Full customize is not always necessary. In fact I'd say it usually isn't. Some cars just have weird gearing and will benefit from it, though. Basically if 3rd and 4th gears are too narrow you're going to run into issues, same if they're WAY too wide (though a little shouldn't be an issue).
I generally prefer 5speeds over 6, but the gearing on the S13 was starting to annoy me a little. I went up to the non customizable 6 and everything sorted itself out.
Hopefully on the February patch we're going to have completely tunable gear ratios, no guarantee obviously but the man himself said it would be added in soon. With this you can focus more on the specific gears you're using to drift rather than just adjusting final drive and hoping everything falls into place. It'll help to set up for specific techniques/driving styles on specific tracks.
TL;DR, no, it's not really that important most of the time. The semi-racing or whatever should be fine if stock isn't working out.
