Well you may get away with 85mm with your current lens, however the Big Stopper is 100mm anyway.
Even though the 10-20mm is a 77mm filter size when you look at wideangle lenses the glass protrudes into a partial spherical shape so you probably won't get away with 85mm there.
What type of filters are you looking to get? I say this because the only filters I will currently use are Neutral Densities (including 10 stop) and a circular polariser, I have ND Grads but no longer use them. Rather than use an ND grads I bracket my photos for example with a sunset I will take one shot exposed for the sky and one shot exposed for the foreground and combine them using a layer mask and the gradient tool in PS Elements, this really is a software version of what an ND Grad does you also have the added benefit of being able to brush out buildings, trees, hills etc where the grad filter would normally darken them. The bracketed shots do need to be taken on a tripod of course to minimize movement, this technique will not work well with moving subjects of course.
There is nothing wrong with using ND Grads I just thought I would mention the alternative which I prefer.
The trouble is with filters is they are a lot of expense a set of .3, .6 and .9 ND's is going to set you back £90 if you use hi-tech and £210 if you go for Lee's.
The Lee holder (foundation kit) is another £48 and then you will need an adaptor ring (wide angle) which is another £31.
So it is quite an investment, you could of course compromise and go down the 85mm route just to see how you get on but then again you could end up having to buy 100mm filters anyway.
These are just my thoughts and experience
