I'm with Overload on this one...so much of the whole thing is mental and if you can get past that, then you should be fine. Sure, you're tired now, but just decide in your mind that for 1hr or however long you train that you're not going to be tired, so I say train.
Funny thing, I did chest yesterday, and last night every time I'd try to roll over in my sleep my chest would cramp up where the pec/delt tie in is. So I did not get much sleep at all last night and I have back training tonight as well. I could skip out on it and push it until tomorrow, but if you do that everytime you're a little tired or don't fell 100%, then you're going to miss a butt load of training days. Again, like Overload mentioned, these are the days that count...countless people hit the gym when they feel like it, and countless people look the same day in and day out.
