Assuming you're not the choreographer:

Should I give the best effort I can when I am practicing a dance with my group? I know what you're thinking: "Of course. We need to be able to finish and polish this dance!"

You gave it more ENERGY. Great. But there's seem to be no sign of a break anytime soon yet. You are able to follow the steps, and you make a few mistakes, if not none. So you asked yourself, "should I still give my best even though I already know the steps and polished myself? I mean, I will still dance, but with lesser effort so that I have less fatigue."

The answer is, still, yes. Unless your choreographer is abusive (though I would leave the group if I were you, or if it's for school, I would complain to my teacher), you have asthma (why would you even join a group dance), or you are really very fatigued and you will faint if you continue giving your best (you should really take a break, your choreographer will understand), continue doing your best. You will not regret it. Don't think about the fatigue. Enjoy it. Personally, whenever I give a practice session more energy, I enjoy it more. That's because physical exercise actually activates the endocannabinoid system, which contributes to euphoria. It also plays a good role in memory, so if you activate your endocannabinoid system via exercise (which in this case is dance), you strengthen your memory. You can remember the steps more easily, no matter how long the dance is. And most importantly, you can minimize your mistakes.

So don't settle for less. Always strive for more. You have nothing to lose but your weight and laziness. If you truly love what you are doing, you can do it better with more effort.

Always remember to take a break though if needed. Don't push yourself too hard. Know your limits.