Individual rifles will vary: Do not presume you "need" to load bullets hot in your new rifle just because someone else found that to be the case. In fact, the conventional logic is that the most accurate loads, with slower powders in the 243, typically come at 1 or 2 grains below max. Consult a recent reloading manual and stick with documented loads.
Most every barrel will shoot better with a little fouling than they will with a clean cold bore. Shooting for groups, with a clean bore at the start of every group, is usually going to be an exercise in frustration.