Well it depends technically.So a dirty barrel is more accurate than a clean barrel? Serious question.
Many benchrest shooters scrub clean before shooting for record (or have some sort of routine that involves deep cleaning). Guys in other competitions do too.
They also clean between strings so as to keep their barrel consistently clean.
While I don't compete in any way, I prefer consistently dirty. It lasts much longer and is easier to maintain. I also prefer to spend my range time shooting, not cleaning.
It is hard to argue accuracy against what the benchrest shooters are doing as far as pure accuracy goes. Keep in mind they spend a lot of time and money to be competitive.
If you want the absolute best group then follow their lead. If you are okay giving up a bit of accuracy to be able to shoot all day and not have to scrub your barrel multiple times then let it stay fouled.
I have shot my fair share of 1/2 MOA or less groups with fouled factory barrels. That's good enough for me to go out and have a great day at the range.