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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.

https://steemit.com/witness/@netuoso/witnesses-in-top-100-on-old-hardfork-versions-of-steemd-and-their-voters-1515951774

But I keep a few open to add new ones that look promising, impress me, or I've just overlooked.