I restart if it's really important or one of those battles in which everything that could go wrong does...but sometimes I'll let it slide if I take out so many enemies that I can quickly mop them up later (examples below)
This one was just two units of cavalry coming to reinforce my forces sieging Damascus getting ambushed by some brigands (I like how rebels actually attack you in SS, even though it screwed over poor Captain Guy here)
And this was just me being a bit impetuous
(The heir was the sole survivor of the battle)