I've done it both ways. Running a race in WDW, I put "it's my birthday" on the front of my DH's shirt. Everyone said I should have put it on the back so they could read it (as they passed us of course).
I think folks spend more time reading when they don't see your face (so put it on the back) but I loved seeing everyone's reaction to my design (so put it on the front).
There really is no right or wrong way; whatever you like - go for it - it's your creation!