I sure hope you do have a written policy against tattoos for your sake; if you're in the U.S. You're walking a fine line and as a hiring manager you should know that. To say you won't hire but there is no policy opens you up for all kinds of hurt if you ever do stick to your guns. I'm sure your legal department has it all covered though.
As someone who has hired folks in the past, our policy is written (because we're smart like that) and the tattoos cannot be visible. If it is casual day then you're bound to wear something that covers the tattoo. I do work in a professional office where the majority of the people employed do not meet the general public face to face. The policy is still in tact for whatever awesome reason.
My opinion is that if it can't be easily and comfortably covered then don't bother. I want a tattoo on my wrist that can and will be covered by a watch. I find neck tattoos and titty tattoos to be trashy beyond belief. I also seem to find a tattoo on the upper arm of a female to be weird. To me, most tattoo locations for males don't really phase me unless they're on the neck, face/head, or ankle. As for the ankle I just find that feminine. These are my own personal opinions that aren't qualified by anything substantial enough to suggest that they matter.
A forearm tattoo on a man isn't terrible but again, see the above. I also don't associate tattoos with drug use, criminal life style or anything like that. I do associate the nature of the tattoo with those things as I believe many people would. If you have 18th street in old English lettering on your back I'll assume and probably be right, that you're a member or former member of the 18th street gang. Such as it is, choose wisely.