It depends on the price difference (and item selection difference) between online and your local B&M, how many dead pixels you're honestly willing to have, and the manufacturer's dead pixel policies.
Online places tend to be cheaper, with a wider selection, but it means means you won't be able to check out the monitor beforehand for dead pixels -- it's basically gambling. On the other hand, even if you checked out a monitor beforehand at a B&M, there's no guarantee that dead pixels won't develop within a week or so, since some don't go busted until the monitor is fully warmed up, and when you check at a store, you probably only waited a minute or two. So going to a B&M isn't a guarantee either. But at least you can eliminate all those that have readily-visible dead pixels.
Sure dead pixels are annoying, but you really have to ask yourself how many you're honestly willing to tolerate. I'm not a hardcore gamer and I tend to be on the philosophy of "well if it works it works", and my last LCD (from 2001) had 2 dead pixels and it didn't bother me. In fact I recently gave it away to my friend, told her it had two dead pixels, and she couldn't find them at all (even though I told her where they were), until we tried a black screen. So it's really up to you.
HP gave some stats in 2003: 60-70% zero dead pixels, 10-20% one dead pixel, the rest more than that. Through statistical modelling, and given some advancements in quality control, nowadays the likely chance is about 80% zero dead pixels, 17.9% one dead pixel, 2% two dead pixels, and 0.1% more than that (those percentages are actually sort of "made up" because I assumed 80% no dead pixels as an input). So you can see if you're willing to take that kind of risk.
Most LCD manufacturers get the LCD panel from the same manufacturer (there are only a few LCD panel manufacturers worldwide), so the quality is actually fairly similar. I personally use dead pixel policies just to see how many returns a manufacturer is willing to take and how much it is willing to deceive customers by marketing-speak that gives you a lot less than it sounds. For example, some say "zero dead pixels allowed" -- then define a "pixel" as RGB subpixels side by side, which is the true definition but not the one that people think of when they see a bright dot on the screen. This one in particular annoys me because my company had been offering it ever since 2000 via our adjacency criteria (but not pointing it out as such specifically; we look at defects on a subpixel basis, and any 3 within 1 cm of each other is grounds for replacement), but it's been in vogue in the past few years to say something like that; HP in 2003 and ViewSonic in 2004 for example. In reality, panel defects are on a subpixel basis, making whole pixel defects extremely rare. The opposite way is to say something like "only 3 defective pixels per million pixels allowed" -- and then define a pixel in terms of a subpixel. This sounds good (it sounds like only 3 allowed for 17-inch and 19-inch monitors) until you realize that there's actually almost 4 million subpixels on an SXGA (1280 x 1024) screen, so it actually allows up to 11 defects. So I would personally say, use the dead pixel policy to consider how forthright and honest a manufacturer is going to be with you, not the actual quality of the panel itself.
Chuck Hsiao
Amptron
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