is it necessary to prewash ribbon if sewing it on to a washable garment?

I've forgotten whether it was a book or movie or what: An elderly cat was passing on the rules of felinity to a kitten, and said that the most-important rule was "When in doubt, wash."

If the ribbon shrinks and what it's applied to doesn't, you will get puckers. But it could look deliberate, as if you'd eased the towel onto the ribbon to improve the fit.

Since ribbon is usually bright in color, it might be wise to check whether it bleeds or fades.

If there is so much ribbon that washing it would be a royal pain, you could wash a carefully-measured sample.

The last time I washed a sample, I basted marks one yard apart around all edges, to make a removable mark that would stand up to washing.

I would say yes wash it first to avoid disappointments, shifts of colour etc