i got to disagree, i dont like the fiberglass reinforced, it is so thick. i use my really thin suckish cut off wheels cuz they are thin. and what type or dremel do you have? 400 xpr? i do. i use those wheels on speed 6 when i cut my i think... 2 mm styrene. if that doesnt work, use the heavy duty but not the fiberglass ones. and as a general rule of thumb, dont press to hard. now some pressure must be applied, but dont just like RAM it into the mask.
another thing. ya, dont cut right on the line. cuz the wheel will cut off a little too much. cut "before" the line and then grind it down nice.