Nope Hamish is right.
A spacer doesn't change the offset, it simply move the entire rim outwards. Think about it, you are in essence adding length to your hub.
The only time offset alters brake clearance is if it starts to change the spoke angle. Ie on a TE37 with a high offset the spokes are flat, however when you go to a low offset the spokes start to bend outwards.
So to answer OP's question, you will be fine. I have GTC's and they don't even come close to hitting the brakes.