Recently rooted about a month ago and I recently heard that since I rooted my MAXX HD, I am off the list for further firmware updates from Motorola. Is this true??
I am going to go out on a limb here and say that you'll still get updates (after you first un-root), but truth is...rooting is against the policy of Verizon/Motorola and they don't have to support the phone once rooted. Also, even if you unroot, they can still tell if you've been rooted previously and again reserve the right to not support the phone.
Still, if you unroot, as it has always been in the past - updates will still push out to you and you can install them successfully.
As you I'm Rooted too, best way to get Updates is to make sure that your device will receive them. To check I suggest going into Settings> About phone> System updates> (should receive a message like the screenshot below). Motorola is known for performing something known as a "soak test" which these are the pre-release test of the update that are sent out to Users> like yourself and myself that register through Motorola's website and opt to receive updates. When a soak occurs and there is actually a soak coming through shortly on the NON-HD Razr and Maxx devices, this is a chance for those registered to try it out, (I have not heard what this update will be, so I cannot comment.)
I also recommend that if Rooted or Un-Rooted do not disable or freeze the apps that refer to "Motorola Services" or "Updater" as these may have a negative affect in receiving any updates.