Just a quick note to add on to what CNEDEER said; If you get guaranteed ME "A" School you'd only have to worry about possible attachment to deck or engineering while waiting for school. Once rated you'd be placed in an ME billet and your main job would be law enforcement.
As Forerunner said there if you join the CG there will be "grunt work" but it won't be any worse that the first couple of years at a police dept. working under a FTO. It's not really "wasting your time." No matter what job you take up, you'll always have to do stuff that doesn't necessarily include Law Enforcement.
Having said that, if I were to join, is there a guarantee that I could get into "A" school, after the wait time and so forth?