Na, dont worry. Little Heat, plastic spatule, isopropyl alcohol (if u have time and u want it perfect, and dont know how the car is painted). If its a modern car, you can use solvent at the end (for going faster) if final coat is ceramic (try to scratch it with the nails). Anyway isoprphyl works always and cleaner (altought slower)
... Little heat... Dont go fast (much heat) or paint we will delaminate.
And yes, always say your customers to remove the magnets from month to month