I've had some of my all time best workouts while being on the tired side.
A good idea is to do your warmup, and if you still feel tired, pack it in or do a reduced volume/intensity session.
But many times your warmup will shake the cobwebs out enough so that you can have a proper session.
Thing is, you're not always going to feel 100% every time out. If you're only slightly tired, suck it up and train.