Speaking as the mom of 3 Boy Scouts (Eagle, Star, 1st class), currently a Cubmaster and Webelos Den Leader, I think it is possible to overprepare Webelos. I bridged the Web IIs in my den in March over to the troop where my sons are. I felt they were adequately ready for the troop. My oldest son was their den chief, and in January of the Web I year we opened den meetings with the Cub Scout promise, etc and closed with the Boy Scout Oath, law and outdoor code. This way, the Webelos had an understanding of the Boy Scout stuff needed for the Webelos Badge, and by sheer repetition, left the pack knowing the oath, etc. From the feedback I received from troop leaders, this is greatly appreciated. I have sat several BOR for the troop and was appalled to find that there were Scouts aiming for Star that couldn't get the oath right. My son aged out before the Webs bridged and then served as Assistant Den Leader. Before the final pack meeting, we gave each Web a mini-board of review for the AOL. (That way they can't all answer at once.) We felt they were definitely ready to bridge. But there really are experiences that new scouts should only have as a Boy Scout, especially with camping and outdoor activities.
Good point though, about picking the correct troop, it makes all the difference in a scout's experience.