I used to, but now I realize it vastly diminishes my libido . I think I read somewhere that your body releases testosterone for each erection.  So the more erections you have before your session, the less testosterone you have for gaining good erections in your session.. What do you think, hogwash?

I never thought about the physiological basis for it, but the less I think about the pleasure I'll soon be having, the more intensely I experience it. While I'm driving to a session, I usually listen to NPR or audio books of the least sexy subjects possible, especially history. As long as the provider is hot, I have no problem getting it up without any advance prep.

usually the provider has it running when I get to the incall, but I DON'T watch porn before a date.

I've watched porn before a session.  Never noticed a decrease in erection, as long as I did not jack-off to the movie.  
But everyone is a bit different.

I've taken my laptop to show a provider a particular sex act in a porn scene so that we could reenact it together. Beats listening to NPR together.

Hahaha, I visited a lady who had 'Joe Dirt' on the tv for the session and never bothered to turn it off... If that doesn't get you in the mood, you're normal...