I agree with swilow. Daily smokers are the only people that should really fux with smoking weed on LSD. I smoke daily, and it hardly even alters the trip for me when I smoke -- I just enjoy the taste, and it helps ease some nausea that can be caused by the insanely high dosages I'm inclined to take.
However, last time I tripped, my friend who doesn't smoke much started panicking MAJORLY after taking just one baby rip of chronic out of my bubbler. It was not a good experience for him; I gave him a temazepam and put him to bed and all was cool, but it kinda fucked up the good vibes during my peak. I didn't mind though, shit happens when you trip--you gotta learn to go with the flow.
edit--on an interesting note. being a daily smoker, I've found that when I take Lucy, my trip sometimes won't really "kick in" until I take a hit of weed. Its as if the weed runs through my system instructing the LSD to stand at attention and go full speed ahead. Often, i won't feel it at all--until I take just 1 hit. Then the air develops a deep, hovering purple fog and I know I'm in for a good ride.