Good question... A lot depends on the quality of the competition. In the higher levels there are usually few secrets. On the local level there are even fewer. If you fish essentially the same lake all the time, the better holes are well known and usually even have names... The Swirl Hole, the Lunch Box, the Snack Bar and etc. Does this mean if your friend fishes one or several of these spots in his prefishing, that you are then prohibited from going there... for how long?
I fished a tournament years ago on Rend Lake Il. I had several holes all lined out and the plan was to hit as many as I could. The second spot I stopped happened to be a spot that the coangler had pre fished with his buddy. He became so upset when his friend saw us there, I thought he was going to cry. Truth is I had found that hole YEARS before and even had won a tournament there.
The best idea is to beg, borrow, or buy your own rig (even if it is a 10 horse on a Jon boat) and do your own pre fishing. The first few years of tournament fishing, I would take a buddy with me and I would find enough fish to do well in the tournament. The problem was we would wind up sharing my fish and neither of us would make any money. I quit taking him along and I finished in the top 5 in two divisions of a well known circuit that year.
After that I actually bought a Jon boat and woud often prefish in it. Nobody paid any attention to the perch jerker!
Hope this helps.