The choice of whether to get a tattoo, where to put it and what type are entirely personal. Here are some medical thoughts to keep in mind.
First and foremost, any piercing of the skin must be done in a sterile and clean manner. Needles should be used only once and for you alone. The coloring used should be approved, verified not to contain known toxic chemicals and in small enough containers that it is used only for one person. Anything that is reused must be cleaned appropriately. A reputable tattoo salon will not hesitate to show you how it maintains sterile techniques and explain everything to you. The artist should wash before working on you and wear gloves.
In terms of the tattoo itself: since your first line of protection against bacteria getting into your body, your skin, has been pierced, there is always a small risk of infection which is why you need to care for your tattoo properly and follow the instructions you've been given.
The risk of any disease being transmitted by the needles etc should be reduced by the sterile techniques I mentioned above. You do run a risk of reacting to the dye. Allergic reactions, while rare, can happen.
There is a recent medical report showing that in the area of the tattoo you may sweat less and the concentration of sodium in that sweat will be increased. For most people that will not be a concern but if you are covering a large surface area and will be doing heavy work in the heat that could theoretically get you into trouble.
Depending on what's in the tattoo ink you may not be able to undergo an MRI. This is an imaging technique that uses a magnetic field and if the coloring has certain metals in it that might create the problem.
Also tattooing in certain parts of the body might block us from doing certain procedures. For example, if the tattoo covers the area of the lower back where we would do an epidural or a spinal for anesthesia we might not be able to.
Lastly tattoos have been reported, rarely, to mask the appearance of skin cancer.
At the end of the day getting inked is a personal decision. Remember that even with the latest laser technology we may not be able to remove your tattoo and there are some medical concerns that you should take into account that I've outlined above.