I wouldn't take a big machine just because it is too much trouble to lift and haul around. A small machine like a Janome Gem or (my favorite) Featherweight would be fine as a carry on. I agree that you should buy the early boarding option so you guarantee you have enough space. I carry my Featherweight in a small soft tool box from the hardware store that would certainly fit in an overhead bin on it's side, and probably would fit under the seat too. If I was traveling any distance, I would pad it with clothes/fabric just so it didn't rattle around if laid on its side.
I travel a lot by airplane, and the one piece of advice I would have is to make sure you can take care of your bag yourself. A heavy FW original carrying case (for example) has a small handle that would be a bear to carry through a big airport. Use something with wheels or a shoulder strap if at all possible.
Pam