I have heard both schools of thought on accuracy shooting with a clean barrel and shooting with a un-clean barrel. The folks who dont clean their barrel before sighting in their rifle believe that you dont hunt with a perfectly clean barrel every time so you might as well sight in with the same conditions during a hunt. The other camp believes that you should do a cleaning swipe through the barrel after every shot ensuring consistant accuracy.
I will share a recent experience when I put this to the test at the range a few weeks ago. I was using my Weatherby Vanguard in .300 WM with Barnes 168grain TTSX bullets. I shot 3 shots at 100 yards with an un-clean barrel from last outing. I marked those shots with blue felt pen. I then waited a few cool down moments, gave my barrel a good cleaning, and shot 3 more rounds at 100 yards. I circled those shots out of a clean barrel with red felt pen. You can see the difference not only in the tightness of the clean barrel group that it makes a difference to clean your barrel after shooting but it also placed the bullets in a slightly different location on the target. Note that this could be shooting error as well as I was shooting off a front rest only but I do not think it is a coincidence that the clean barrel shot the 3 bullets within 1 M.O.A. Try it for your self and see what happens.
If you want to watch a video on cleaning your rifle check out:
http://www.huntingtipsandtricks.com/p/How_To_Sight_In_A_Scoped_Rifle_