I am installing a new 42" x 60" shower with sliding glass doors. To make it easier to turn on the shower before getting in, I was thinking of installing the shower valve close to the doors and centering the shower head. Would this look odd? Would this make it awkward to ajust temperature/flow while in the shower?

Having the valve on the same wall as the shower head and having it offset, might look a little odd, unless you place it so it appears to have been done for a reason, such as turning it sideways so the cold is above the hot, which would make both handles equally accessible. Or putting the shower head on the back wall. If it is a single handle shower valve, then it looks best centered. But one thing you might consider is to install it as a tub/shower diverter setup with the spout about 18" above the floor. Then you can turn on the shower without getting a burst of cold water and once the water temperature is adjusted, pull/push the diverter and use the shower. This also has the advantage that if you do not want the shower for a moment or two, you can turn off the diverter until you want the shower water running again without affecting the water temperature.

i always recomend that the valve is offset to the front of the shower. it forces the user to stand to the back and the water splashes to the back minimizing possible leakage around the door...

I offsetted my shower valve in my home 6" to the entrance side just as you mentioned. Look up info on ADA requirements for shower and tub valve locations for ideal locations. GOOD LUCK