I'd say it's not necessarily a big deal one way or another. I prefer mine running on a separate machine than the Tableau Server cluster. The reason is primarily because VizAlerts, by virtue of how it works, is a good tool for letting you know when there is an issue with your Server. If something happens to your Server host, VizAlerts will start squawking. It also will consume some level of system resources, but probably not enough to care too much about.