Depends what you are injecting. I like to inject so as to minimize sleeplessness and maximize the chance of high blood levels during my workouts.
Long esters (EQ, Deca, Cyp, Enant, etc.) can be injected any time since they hit so slowly.
Very short esters such as Prop are best injected at night (prop starts to "hit" in about 14 hours - a moning injection can wake you up with heart pounding and raging test 14 hrs later at 2 am, an evening injection will give you high test levels just in time for your workout the next morning).
Suspensions such as test susp and winstrol are best injected in the morning, as they take effect in only a couple hours (or 2x daily if you want to smooth out blood levels).