As for the OP, I wouldn't say that is cheating. However, if you feel it is an exploit, you can make a house rule about not doing it to make things more of a challenge.

For the bridge battles, that really isn't a cheat either, but I try to avoid it. Sometimes I will purposely do it, though. Like when an enemy won't stop sending stacks after me and I just want to end the war. It is a whole lot easier to not have to fight a real battle every single turn.