Like everything, optimization is a complex topic, so I'm going to simplify it into three easy steps:
- Find the problems ("profiling")
- Sort the problems according to benefit-of-fixing/cost-of-fixing ("ROI")
- Fix them, in the order of highest benefit/cost to lowest
Most programmers probably think of optimization primarily in terms of CPU profiling, but it really applies to everything. (and in fact, this post is about the general process of optimizing, especially for things other than code -- for specific cases of hardcore game engine optimization or whatever it might be totally wrong)
For example, people who continually "pinch pennies" (perhaps driving around town to save a few cents on gas) but then waste thousands of dollars on something stupid like a fancy car or credit card interest are probably not optimizing their spending very well. They are putting most of their effort into low reward activities (saving a few cents on gas) while losing much more money in other places. Big, dumb, Dilbertesque companies are also notorious for this kind of thing, cutting back on pencils while running super-bowl commercials, or something.
This is also the kind of activity that I was talking about in my post titled "Wasting time on things that really don't matter". It's not that there is zero difference between two slightly different ways of testing for the empty string, it's just that your product probably has 100,000 more serious problems, and so time spent worrying about the empty string style is time NOT spent fixing those more important problems. Put another way, fix all of the other problems first, then debate the empty string.
Of course you may enjoy debating the empty string (and in fact there are some interesting arguments), and that's fine. The zeroth step in optimization is to decide what you are optimizing for. If you're looking to maximize the amount of time spent on endless debates, or perhaps code "beauty", then maybe the empty string debate really is the highest benefit/cost activity (because the benefit is so high). If, on the other hand, you are trying to maximize the success of your product, then the empty string debate probably has negative value and should be strongly avoided.
The empty string comparison is obviously a small detail, and it's tempting to think that we should not bother with such small details, but that too is a huge mistake. Another important lesson of optimization is that some small fraction of the details will determine the majority of the outcome. This is commonly referred to as the 80/20 or 90/10 rule, stating for example that "90% of the time is spent in 10% of the code", but it could just as easily be 99/1 or something like that (and for code that has already been well optimized, this 80/20 rule may not hold -- it's just a heuristic). In code it may be just a few lines in the "inner loop", or perhaps it's the interest rate on your mortgage ("what's a few percent?"). I once profiled a server at Google and discovered that something like 90% of the time was being spent on a single line of code: a call to sscanf (repeated millions of times) that someone had snuck into the scoring code (that's the 99999/1 rule).
Determining which details matter a lot (and which don't) is also the key to designing successful products. In the classic slashdot iPod post from 2001, they summarized the newly released iPod thusly: "No wireless. Less space than a nomad. Lame."
History suggests that those weren't the important details, since it's 2007 and there still isn't an iPod with wireless (not until the 29th, at least), and most iPods have less than the original 5GB of storage. Apple did, however, get a few very important details right: it's easy to get your music onto an iPod, it holds "enough" music (unlike the early flash players), and it fits easily into your pocket (unlike the early disk-based players).
So how do we determine which details are important? Observation. (and some judgment, but I think that's mostly formed via observation) If you're optimizing an application, observe which resources are being exhausted or causing the delay (it's not always CPU -- it's often disk seeks), and then observe where they are being consumed. If you're trying to optimize money, first observe where most of the money goes to and comes from. To optimize a product design, observe which things are most "useful", and which are most painful or annoying (waiting or thinking, usually).
If you get all of the important details right, you'll be so far ahead of everyone else that the other details probably won't matter. But remember, your time is finite, so time spent on unimportant details is time not spent on important details! Most engineers are very detail oriented people, and this is good because they can really focus on the important details, and it is bad because they can really focus on the unimportant details! I suspect that learning to tell the difference will be worth your time.
By the way, to get even more details right, decrease the cost side of the benefit/cost equation so that you can do more things in less time (by finding easier solutions, avoiding endless debates, etc).