I usually approve them given that the intent of the edit is to improve the quality of the post and that they've made a reasonable effort to correct all the issues with the post. I don't see it as a waste of my time, we're a relatively small community and suggested edits are sparse.
In cases where there's another spelling mistake or two that I see, I usually "improve" the edit. I don't expect someone to be perfect and catch every little minor spelling or grammar error, so I'll give the benefit of the doubt.
When there's a negligible edit on a post that clearly has lot's of other issues is when I'll actually reject it as too minor.
My rationale behind this is "I do it, why shouldn't they be allowed?". I'm sure you could go through my edit history and find bunches of 1-5 character edits to posts correcting spelling or grammar. Sometimes there just isn't anything else to improve upon, but that doesn't mean we should allow the errors to remain in place.