It depends on your risk tolerance and how many DAU/WAU/MAU you have (and whether you're willing to have an untested experience for X users.) For example, I would test really old browser versions, and if there is a bad experience, you could let the user know to use a different browser or to upgrade.

My intuition says you'd get better value out of testing different browsers shallowly rather than 50 old versions of Chrome, though.