I normaly do, that way the main sets feel much lighter. You don't need a whole lotta reps just enough to signal the brain and CNS about the weight.
I normally do 10 reps with a very light weight - ie just bar, then add 25% of my work set weight and do 8 and then add 50% and do 6 reps and then 75% with 2-3 reps and then start