Honestly, most people don't put materia into gear as they level. Most of the stats you'll actually care about will be capped on your armor and weapons if you get a HQ item, making materia just for tertiary stats that might be nice. It's mostly a concern at endgame, where you might want stuff like +crit capped on ALL armor, not just the ones that happen to have it, or accessories with both caster stats AND vitality.

Of course, if you're converting your old gear when you're done with it and thus getting tier I and II materia (Not that valuable to the endgame crowd), feel free to have someone meld it into your gear. Just take care to have it done on a piece of gear that doesn't already have that stat on it, since the existing stats count towards the stat caps.

Oh, and I usually tip 500 gil.