Final Fantasy I - No Walkthrough needed. All areas (except one underwater I believe) can be visited again. You can check walkthrough after you are done to see if you missed anything.

Final Fantasy II - Same as 1 except there is a couple places you can't go back to. The emperors ship is one to mention on the top of my mind.

Final Fantasy III - As far as I remember I used one just to make sure I got everything. Never played it enough as I didn't actually own it until recent.

Final Fantasy IV - Everything can be re-explored for the most part. Some areas are. maybe just look online for missables and your good to play.

Final Fantasy V - I have never used a walkthrough for this game. I looked some stuff up but never had problems finding things.

Final Fantasy VI - Totally use a walkthrough. Some chests shouldn't be opened right away some should. Some espers can be missed, as well as important decisions on how to recruit characters.

Final Fantasy VII - If you have enough sense to examine something that shines you are good to go! However a lot of people have trouble finding things and a lot of materia can totally be missed.

Final Fantasy VIII - The game is easier if you don't level up? Ok. Some cards can be missed. Ok. Rare items are difficult to find or make. Sure looked like I needed a walkthrough here.

Final Fantasy IX - Not really needed but you will find it fun to collect everything. I didn't use one and managed to get everything.

Final Fantasy X - I owned the walkthrough but the only important stuff was by completing crazy quests.

Final Fantasy XI - One month free trial. If you are not a good mmorpg'er look some stuff up. This can be hard to do but not much is missable.

Final Fantasy XII - buy the walkthrough. You will find that you can get everything in the game within 100 hours. Everything in this game besides basic story is a secret.

Final Fantasy XIII - A lot of areas can't be visited again. Miss 1 thing and you can't get it again. I'm looking at accessories. I did it without a walkthrough but for completion sakes I used one on another playthrough.