Malamir,
Good question. While there can be some down side risk to over-practicing, the better you do at a tournament and especially if you're in multiple events, the goal is to play a lot of foosball. Conditioning outside of table time is a good idea for improved stamina. Getting the muscle memory grooved for each shot can decrease fatigue. The only correct answer is what works best for you, and a large part is getting the most out of your practice time.
Pretty generic reply, I know, hopefully others will add more.
Take care................................Tyler