Todays electronics can take a range of frequencies and are built to a global market. If you are a couple of Hz off of 60 you are going to be fine. Same with voltage, it can take a little swing. I have said it before, with today's electronics total harmonic distortion is the big thing. As said before in this thread, your sine wave can be a little off and you will be OK but if it isn't a sine wave or you have multiples of that sine wave embedded in the main signal, that is when you are going to get into trouble.