If you can avoid the very last row with a carseat, it is best to do so. This is because you might need to get into the seat behind yours if the seat belt's buckle winds up wedged in too tight. (Putting a twist in the strap so that the release faces down usually helps, but sometimes it doesn't work, in which case the easiest way to release it is to go around behind the row and reach UP between the seatback and seat to release the buckle, preferably while someone else puts some weight onto the seat from in front.)
It is a very good idea to print out a copy of the federal carseat use rules so that you have them in writing when challenged. I always kept a *current* printout in an envelope taped to the back of the seat shell. (This was more important when travelling outside the US; foreign employees of US carriers most often are not up on carseat rules.) Obviously, you would want to turn these over very politely and in a non-confrontational manner. The rules are in the Code of Federal Regulations, 14CFR121.311 http://a257.g.akamaitech.net/7/257/....gpo.gov/cfr_2006/janqtr/pdf/14cfr121.311.pdf
Note that those rules do NOT mention which rows (except for the exit row) where seats may not be placed. That is determined by airline policy, and is in the flight attendant's safety manual. If your FA is determined not to look at the manual to check, there isn't much you can do but accept whatever seat you are moved to.
Another tip is to request a seatbelt extender from the FA. Using that makes it MUCH easier to install/uninstall your car seat.