I put 10%. If there is occasion for the players to break into a guarded facility, I might use a trap or two, but I don't have many random underground hideaways full of treasure and traps.
I don't really like the 3.5 trap rules. Most traps it's either going to kill the PC or it's not, so they're only meaningful if combined with some other danger. And if you calculate prices, the people who make these traps aren't getting much bang for their buck.