I wouldn't expect the coffee grounds to cause a problem in the trap at all - there's plenty of water flow there, and some vertical head from at least the sink basin down (and more if the sink get filled higher once in a while, and drained fast). The water is moving relatively fast through a 1 1/2" pipe.
It's when those grounds hit the 4"~6" pipes that have just a little slope to them (1" in 8' is the code IIRC). Now that water is moving much slower, and those grounds have a chance to fall out and build up. A little at a time, day after day, until one day they cause a back up.
Might never happen, but I wouldn't want to chance it.
-ERD50