If you do use tape, get good tape. Don't cheap out. I agree with Chad: Frog Tape is good stuff.
The reason I say not to mask off is because the tape will have trouble sticking tightly to a rough surface like grout. You will probably get paint seeping under it. If I remember right, the yellow from tape is lighter and maybe more pliable so it may do better. If you cut in, it might get up on the grout a bit but I think it will do that in a uniform way and look ok.
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Paul
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