Used to, but the scores weren't much different. Sometimes the practice card was better, sometimes worse. Gave it up last season because I had too many cards and not enough time to shoot practice cards as well.
Came out with a 97.57 average across 5 leagues (54 cards). Well up from my previous season's 96.2 avg. Even managed to knock out 4 tons amongst them. Well chuffed.
There's definitely a psych element. I find if I do a practice card, then it focuses (read: distracts) my mind over the fact that my next card has a sticker on it. Having given up practice cards (except in dedicated training sessions obviously), one card becomes much the same as another. They're just cards and I forget about the stickers.
I would advocate just getting on with it. Yeah, you might be a bit nervous first couple of times round, but as long as you concentrate on decent sighters, then after a few times, the stickers vanish from your mind, and your shooting should improve.
"A fear of weapons is a sign of retarded sexual and emotional maturity." Sigmund Freud
Shooting is my meditation