What type of diverter do you have to change the water flow from the tub to the shower head? Most common these days is a lift gate on the tub spout, if this is your case, I've never seen a spout diverter ever seal completely. The second style of diverter that I know of is an actual valve handle usually between or below the main hot/cold valve(s ), if this is your case, then you should get a complete seal between the tub spout and shower head. The latter style valve works just like the hot/cold valves, if it's leaking, then it needs to be rebuilt.
If you've got the latter diverter valve style and it's still leaking after multiple rebuilds, then make sure that you're using the right washer on the end of the valve stem and that the seat is in good shape. If either occurs, then leaks happen, however it should be a minor enough leak that it wouldn't affect the function of the shower.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!