Exactly. Keep a few open always.
You may already be aware but @netuoso keeps track of witnesses that are running older versions (in the top 100) or are becoming inactive, etc. so there's always a chance my number would drop by a few. I take a look at his posts on this every once in awhile to make sure I'm not wasting votes.
But I keep a few open to add new ones that look promising, impress me, or I've just overlooked.