I use filters behind the lens for the simple reason that a good number of my lenses have front elements that are just plain too large for my standard 3X3" filter. Even my 100mmX100mm Sinar filters don't cover the entire front element of my Schneider 5,6 360mm lens. So, intead of carriy several sets of filters, depending on the lens size, I have one set that covers everything from my 250mm WF Ektar down to my 210mm Schneider Angulon behind the lens.

I do my focusing and comoposing without the filter, then place the filter on the back ofg the lens using rolled up tape, replace the GG and then refocus. it's always worked for me.