Shooting a great round is harder for me simply because it takes a much higher concentration level to maintain it for the entire round.
I've always made it a practice to finish every round I start so finishing a terrible round is not difficult at all. If the score is poor I can always take a practice attitude to the finish. Work on certain shots.
After all, a poor score in this game is meaningless. Once you have the required minimum number of scores for your level they no longer matter and are easily replaced.
Plus, if you are playing a round with someone it's a commitment you made when you started the round.