it is important in some ways, and unimportant in others.
Often, a fish profile will say that neons need, say, a pH of 6.4-7.0. This info is often based, however, on the natural conditions taht they are found in - aka where they originate from. The neons that you buy at the LFS are probably farm raised and have been subjected to different water parameters than are found in the wild. That said, most people run on the same water as their LFS, so most fish have been acclimated to your region's pH beforeyou even get them. Neons can actually live in a pH of 8 (not suggested, but it happens)
I say to think of pH like humidity to humans. If you lived in an area where humidity never went above 30%...then moved to a region where it maintained around 90% - it would be a bit of a shock. You'd be uncomfortable at first. But you'd acclimate to it and soon it would feel like normal.
So, pretty much, the thing about pH is to keep it constant. pH really becomes a factor when it is swinging and fluctuating. One of the biggest no-no's you can do related to pH is to try and use a chemical to alter it - such as the pH up and pH down that is in the pH test kit you bought. All this accomplishes is temporarily lowering/raising the pH - the source of the pH has not been altered...and therefore the water will bounce back to the original pH. So you add more chemicals...so it bounces back up...etc. It turns intoa fluctuating cycle. Rather than adjusting a pH to suit a fish, its better to chose the fish that suit your pH
There are ways to better alter a pH, but first you'd need to have a pretty good reason to do it.
All in all, it isn't THAT important - it only becomes a problem when chemicals are involved, when you're trying to breed fish, or when you are trying to keep VERY pH specific species. Or if your pH is below 6.2-4 ish or above 8 ish. Even then, many species are capable of living above/below these boundaries.
Hope that helped some...