I try to train as heavy and as hard as possible at all times,in the event that I feel like shit or an burnt out,I just lighten the load and rest less between sets to make up for the lack of poundage.
I never have a bad workout if I train like this...bottom line is,listen to your bodys feedback and train accordingly.
Every day can`t be a day of new personal records,but if you know what your doing,everyday you train can be a great training day.