Truthfully, it's not a big thing to worry about OP. tl;dr of what's below: Use whatever the most specific*, highest iLVL is available to you.
(*Specific?)
By most specific, I mean gear specifically for BLM/RDM/SMN/BLU is better than one for all "Disciples of Magic", or gear for Tanks is better than something for all "Disciples of War". (See the level 32 Calvary vs Infantry set, for an example.
Lots of text, but I suggest:
1) Getting By: Get by on just using whatever drops from Dungeons. There are some great sets you can keep for later jobs as well (the 17 sets from the first dungeons, the 32 sets from Brayflox's Longstop dungeon stand out). Certain "colors" of items will be better than others...GENERALLY speaking Gray < Pink < Green < Blue
2) "Expansion End" Levels: With each "expansion end" (50, 60, 70 and now 80) there are a bulk of iLVLs available. Most of them can be ignored; You have several options for decent gear:2a) Tomestones: At each of the "ends", the highest Tomestone set should last you to about 5 levels into the next expansion if you choose to get them. You will naturally accumulate these Tomestones as you do stuff in game.
2b) 18 Man Raids: These are pretty easy at this point and offer decent sets...should last about 3 or so levels into the next expansion. Does not require grinding tomestones, but does rely a little on RNG.
2c) Scorched Earth: Just burn through the highest dungeon open to you and grab what pieces you need.
3) Fill Gaps: Worst case, there is always the market board- just buy whatever the highest level and most specific to your job (eg. Tanks can wear some of the same stuff as a dragoon, which is not as good for the tank)
4) Keep Up to Date: However you decide to gear up, it IS important to keep your gear as up to date as possible; Wearing level 17ish gear in a level 30 dungeon will cause a noticeable drop in damage.
Closing: Don't worry about actual stats TOO much, because you're just going to replace your gear in a day or two anyway. The game doesn't really have lateral progression with gear like some other MMOs. At the end of the game, unless you're doing some VERY specific things, you'll only have 1 set of gear for your job.
Disclaimer/Information: I'm an uber-casual. I'm almost all 80s, I've done almost every quest since launch..but I DO NOT touch the end-game raid stuff until it's old news (still not even done the last 1/3 of Omega NORMAL mode)