My HW77 requires more frequent barrel cleaning than any other rifle I've had. Using JSB Exact it needs a good clean every few hundred shots, the pull through comes out very messy with lead flakes.
The groups open up quite abruptly and close again after cleaning. I've had the gun about 7 years and this has been a consistent pattern of behavior.
On the other hand I did clean RobF's HW97 while it was with me on holiday. Despite not having been cleaned for thousands of shots the pull though came out looking the same as when it went in. The gun was also incredibly accurate before (and after!) cleaning.
Just goes to show that there are no hard and fast rules; you have to get to know your gun and pellets. My personal suggestion would be that, in general, most rifles seem to benefit from an occasional clean.
The only gun I've had which behaved similar to Rob's HW97 was an FWB P70; this grouped amazingly well and the barrel never picked up any mess.