Most bluegrass players use plastic thumbpicks, however, there are some that prefer the metal. I think it's according to the sound you want.
If you can't find plastic thumbpicks, check out www.elderly.com or www.banjostore.com or some of the other vendors on this list. Thumpicks are very inexpensive so you can order a bunch and try them and see what fits and sounds good to you. They DO come in different sizes, so you might try a variety of sizes, too.
Plastic Thumb picks are just easier to use. They are more flexible, and lighter. Also I dont notice the difference in sound. But go to www.banjo.com that is the Best store for bluegrass. they have everything for banjo. I like the Propiks.
i'm sorry- i really don't want to step on any toes but some times i think too many people worry about the things that are least important. there is no advantage to using one pic over another, they both have qualities that the other doesn't. the best thing in the world for making your banjo sound and feel better is to quit f'n with it and play the darn thing. practice is waaay more important than which pic you choose. i don't know about anyone else but I'd rather listen to someone who can play, jam out on a 30.00 kmart special than someone who can't play sweating his way through cripple creek on a pre-war gibson
Jason---The only way he'll know which he prefers is to try them both. No big deal. But in the long run, and as experience begins to build, I'd put choice of picks (thumb and finger) on an almost equal level with choice of strings when it comes to pulling the tone you want, and getting the kind of response you like, from your banjo.
So the original poster asked, "How important is it to have plastic picks?"
I guess the most consise answer is: "It's not important at all. Like your choice of strings and capo, it's just a matter of personal preference and comfort. It's not going to make you a better or worse player".
I switched to a metal thumb pick because I was breaking the plastic ones too easy/fast. The metal ones last longer for me. Other than that, there was no reason for the change. Honestly, I didn't notice a sound change, but I wasn't looking for it either.