Uncaught Error: This socket is closed. MQTT connectivity issues

Is it good practice to execute reset(true) before I upload any new code in the future?

No, you should be fine without it - realistically it's only if you were switching the 'save on send' setting and you were sure you didn't want anything left behind (sometimes you might - eg if you're making your own bootloader ).

After about 3 messages I receive: MQTT disconnected... reconnecting

Ok - so this is unrelated to the recent changes then? What MQTT server are you using? Could you see if you can read the logs for it? It's entirely likely it doesn't like something that Espruino's MQTT client is doing, and so it drops it.