Should I invite my fiance's grandparents to our wedding?

You should be polite socially and that includes inviting them if your fiance wants them there.

You invite them and be respectful and polite toward them. You don't need to be best friends with them or spend lots of time with them.

I bet there might be people in your family your fiance might not particularly like.

Just part of life.

If you're having other family at the wedding, yes.

If you're eloping, no.

About the only other way to avoid it is to have a destination wedding they can't afford to get to (a colleague did that to avoid having a fair bit of his and her families at the wedding)

One other thing though how does your fiancé feel about this? Does your fiancé want them at the wedding?

given the OP's other posts I'd be very surprised if there was a fiance or a wedding