Yes, it is usually allowed. Most of the manuals word the restriction to say that the carseat must not impede egress, so as long as the seat position is the farthest out from the designated exit for that row in either direction, you can usually put a carset there.

Expect an FA to pop beside you and confirm your identity as soon as you begin to install it. For some reason, on long-haul flights they seem to be much more surprised to see carseats than on short flights, and they always want to challenge Jr's right to sit in his own seat. Challenges are especially likely if you install a rearfacing infant seat on an overnight flight, because they would rather not deal with "I can't recline" complaints from the person in the next seat forward.