Hi Chuck, you didn't say what size wheels you are using so I will make some assumptions. Your idea for washers should be fine, just make them thick enough so they won't distort when tightening. Proper practice is to place paper blotters between the washers and the wheel on both sides. These can be made from thick paper or thin cardboard.
Make sure you ring the wheels before installing them. If you take the wheels off for some reason rering them before remounting. They should be lightly tapped with a wood handle, if the wheel rings then it is good to use, if the sound is muffled or dead discard the wheel by breaking into little pieces with a hammer. Cover the wheel with a rag before breaking to prevent flying debris. Be aware of the max rated speed for the wheels you are using and don't exceed.
My experience with grinding comes from working in a bearing factory for 30 yrs. I have seen many grinding wheel accidents and wish to see no more. I suggest making wheel guards, when a wheel explodes it is catastrophic. Stand to the side when powering up and when using the wheel. If you damage the wheel in any way discard it. Our bosses had no problem with us discarding wheels if they were suspect. Alot easier and cheaper than filling out accident reports. Wear your safety equiptment and don't be distracted when a wheel is running. I don't mean to preach but I have been taken to the ER to get stitched up from a grinding wheel. Dave
By the way, I got stitched up by a Dr Payne, go figure.