You can stack up to 100 leves.
If you're 50, leves serves as a way to earn some easy gil and seals (from GC leves)

For leveling, I (mostly) leveled botanist and miner 5 or 10 levels above other craft to be able to gather my own mats.
Then, I leveled each craft simultanously (with some priority to WVR, CUL and CRP with a bit of LTW)

For gatherer, doing daily delivery from GC is a free "leve" for xp and give seals too ^^
(and if you're as crazy as me, do each crafting delivery to fill you're seals as fast as 5k seals a day ^^)