ORIGINAL: Solitary Man
Don't know anything about your trimmer, so I don't know if it's okay to trim them with primers inserted. I would think so, though. I use a Wilson, but have never trimmed brass that was primed.
As to trim length, the manuals suggest a trim-to length of 2.610" for the .300 Win mag. That's what I trim to with mine. Shouldn't be necessary and probably not advisableto trim to 2.600". The neck on a .300 WM is short enough as it is.
solitary is correct on 2.61", but I guess I push mine. I hate trimming. So I always go to 2.60. Starting out, you should probably follow solitaries advise. Later, trim little lower.