So I play support a lot (you could say that I main support). Lately I have taken to filling up on wards if I cannot either finish an item or buy a part without closing my item slots. Is this a good idea? Should I save at least some amount of gold or should I should just stack as many wards as I can buy at that point?

Edit: I know how to build a support; please stop posting your support builds. My question is about specific situations in which there is a decision to either buy an item and temporarily fill the sixth slot or to use that moderate amount of gold to stack wards in that slot.

In early game, try to buy GP10 items ASAP. If you don't have enough money for that, you can buy 3 wards everytime, and then go for those items that can ungrade to GP10 items (e.g. Fairy Charm --> Philosopher's Stone)

[sorry for my bad English lol]

depends on the situation, I never leave the base without a couple though.
If your team is really stomping, just keep key areas warded.
If your team keeps getting ganked, buy more.

I find that I often have five slots full when I recall because I'm building an Aegis or something . A lot of times I have ~400 gold and I am always tempted to get the mr instead of getting five wards.

You should get GP10 runes and masteries if you don't have them already

I think you are misunderstanding and/or misreading; I play support a lot. I know how to build support. That is not what I am asking in this thread.

Generally, when you start off, you want to rush GP10 as quick as possible but you also want at least one ward, preferably two, for gank protection. Just having a ward there will discourage enemy junglers from ganking your lane. Once you get GP10 (usually philo stone and heart of gold), that's when grabbing wards is of more priority than finishing your item build but you shouldn't need more than 5 wards per B.
---
For instance, with Sona I start off the mana regen part of philo stone, 4 wards, and 1 health pot. Most people go 3 wards and 2 health pot but I find the 4 ward start much more effective since I have sustain and the 4 wards guarantee me to stay in lane longer so that my first B to base I can get philo stone.

You should ALWAYS leave a slot for Wards as a Support. No matter what. If you have 400g and 5 full slots, don't buy that MR if it will fill up your slots. You don't need to necessarily spend all your money on 5 Wards every time, but you should grab a couple instead of filling up your slots. Trust me, your team will like map vision a lot more than a few stat boosts!