I think that as long as you complete your exercise session, you can cum and it won't matter.
Blowing your load can be a hinderance if you're cumming too soon into the session, losing your erection, and not giving your dick a chance to get a good workout.
I've tried it both ways and it's the same result. My dick still shrinks up a little after a workout whether I cum or not, so the way I figure, I might as well bust a nut after my workout instead of holding back and getting blueballs. All that stimulation with no payoff can be really aggrivating sometimes.
As long as I get in a good hour or more of Penis Enlargement before I do it, I feel no guilt just letting myself cum. It's like my reward for a workout well done.
I even do more jelqs and manual squeezes after cumming just to make sure my cock doesn't immediately retract. Try it, it works.