Any decent tyre place will balance the wheels by hiding the weights behind the spokes. Thats what my local place does anyway 
Surely they have to put the balance weights wherever the balance point is? Otherwise they won't be truly balanced?
In theory if you add more weight, you can counterbalance weights that have been put in the "wrong" place. i.e. (I'm guessing at these numbers but) if the machine said you needed a 5g weight at 6 o clock, right opposite the hole in the wheel where you would see it, you may be able to achieve the same result by adding 10g at 5 and 7 o clock which would be hidden behind the spokes.
Getting the tyre fitters to appreciate this level of OCD would be another issue tho'. I've only just got mine tuned into the idea of making sure the center cap is the right way up when the valve is in the 6 o clock position.
