Tattoos are not looked at here in the UK for medical information.
If you come into an A&E unit unconcious they will look for a bracelet or pendant as a matter of routine. I would definitely wear a Medalert bracelet or pendant with the following
Multiple Metal implants NO MRI!
Suspected Latex Allergy

Since I have metal implants, I asked my doctors about this, when I had my surgery. If you came into an ER, unconscious, with no medical information, you would be physically examined and X-rayed (or CAT scanned), well before any type of MRI was done, partly to check for medical implants, and also to survey for foreign material in wounds that can't be directly examined.

As for allergic reaction issues, you'd likely be in the care of team with years of practice in assessing the condition of unresponsive patients and supporting life functions. You're unlikely to die in an ER of allergic reactions, if you're breathing and have a heartbeat when you arrive, whether you have a MedicAlert bracelet or not.

But if you want a bracelet, by all means wear one.

If there isn't reliable information on hand that there isn't metal in your body. Or if there isn't an opportunity to conduct a proper interview before MRI, an MRI will not be done.

It's up to you if you want to wear a medical alert bracelet about the latex or metal. In most instances, hopefully, you will be conscious and will be able to alert staff to a latex allergy and metal in your body. I am sensitive to latex also. If I keep gloves on for an extended period of time, my hands are burning. I wouldn't classify this as a true allergy.

As Wilder says, by all means get a bracelet if it will make you feel more comfortable. It can't hurt.

I have fairly severe allergies to most antibiotics, plus asthma. While it's not really the sort of thing that EMTs are likely to be worrying about, I feel better when I have my MedicAlert bracelet on.

I figure: worst outcome of wearing it unnecessarily...um...wrist chafing? Worst outcome of NOT wearing it...sudden death. I vote for the wrist chafing.

I didn't like the look of the steel bracelets, though, so I got my crafty sister to make me a bunch of pretty beaded bracelets in different colors. So the bracelet attracts less attention, unless you're looking for it.