>or have a plugin that rotates it often to counter fingerprinting
That's not going to fool any competent fingerprinting scripts. Blink/webkit/gecko have different javascript implementations so it's easy to cross-check whether your user agent header matches your "real" user agent. The only thing you're really going to fool are server side logs for user agent. If anything, using user agent spoofer is an entropy source (makes you stick out more) because most people don't spoof their user agent.