Dennis, resin is a much softer - yet more brittle - material than styrene. It is FAR more susceptible to easy breakage, warping, etc. Flush cutters CAN be used in some situations that are noncritical. but most times whan I have tried cutting resin sprues with a Xuron cutter, for example, there has always been some amount of small damage to the part itself. Resin seems to "catch" a cut far easier than any other material.
Actually, I do use flush cutters and cut a couple of mm away from the part. Then I sand it down. Also, I use a grinder which is the most expedient way. However, a note of caution: make sure the bits are not dull and the torque is sufficiently high on your grinder (probably Dremel), otherwise, the bit will catch on the part and will "run" across your part leaving these disconcerting tracks.
Sawing through resin sprues and resultant trimming is the most reliable method. I use an X-ACTO microsaw blade in a #1 knife most of the time to do this. To trim resin flash, DON'T use a #11 blade - it can "hook" into the resin and irreparably damage the part in question. Use a ladies' emery board or a hobby sanding stick instead. A Flex-i-File works particularly well for small, tight corners.
I learned the hard way - by my #11 and Xuron cutter destroying a couple of over-$50 resin items . Experience is the best teacher, but that was a VERY expensive lesson.
In these cases, make sure you have Gunze Sangyo's Mr. Surfacer handy. It will fill in these microscopic marks. An example of how well this works, I use 400 grit sandpaper to do the rough sanding. Then I graduate to 600-800 grit for the final polishing. However, the 400 grit will leave some rather deep scratch marks. Not to fear. With Mr. Surface, I just liberally glop it on the area, let it dry, and then sand lightly with 600 grit. The result is a bunch of deep grey lines where the scratches used to be and the whole thing is really smooth. (Depending on how deep the scratch is, you might need a couple of applications).