I've updated my Ultra firmware a couple times since the initial release.
No problems at all. Only the 1st update did something (fixed USALS motor issues), the 2nd one did nothing for me at all.
With that said,
If it's working OK for you as it is, leave it be. If you're not having any issues, let the warrenty continue an go on as is.
I'll let someone else with a
Mercury II chime in if the update fixed anything on their receivers.