From what I've seen by just looking at balls on the rack, most don't. It doesn't mean it's not a good thing to do, but that's what I've observed.
Personally, I do. I like having the option of going to a larger slug size in those rare situations where I swell, and taking out tape isn't enough. It's insurance I'd rather have than not. It also provides a second slug you're comfortable with in case something happens to the slug in your primary ball. This actually happened to me a few weeks ago, and it would have been a really bad situation if I couldn't have used my spare ball slug.
So it depends on your situation. If I only bowled a handful of games a week, I might have passed on it.
