I just couldn't see crossing over in January, regardless of advancement, those scouts are still fairly young. A few extra months being the top dogs around the pack can help build that confidence before they join the troop where they are the little kids again.
Recruiting in my area is done like a fraternity rush. Many troops have used this method for years and aggressively pursued Webelos during the fall. Many times troops will close off recruiting and stop accepting Scouts as early as December. Because my area is so (needlessly) competitive, families buy in to this crappola and sign up early. The irony is that if the troop does not manage the application submission correctly (e.g., hold off submitting applications until the Scout has been awarded AOL) they can actually screw up the kid's advancement.
We take a different approach. We don't set a limit of how many Scouts we will take. We recruit year around, so that we aren't just showing up and asking for Scouts every October. We offer to help with the Cub program, help at meetings and Den Chiefs. This approach has lead to a steady stream of 15-20 new Scouts every year.
I agree, March is the perfect time to cross. No one wants a brand new Boy Scout going on a January or February camp out where they may not be properly prepared.