Just so i have it right; So as long as i do the right amount of weight, it doesn't matter if my muscles FEEL tired or not right after a workout?
No. Focus on the results of your workouts, not how you feel after you've finished one.
If you're getting bigger and/or stronger as a result of your workouts, your training program is working. If you aren't, it isn't. How you feel after any individual workout isn't a good measure of how well your training is going.
EDIT: Doc beat me to the punch, and stated the issue more clearly:
You should feel like you're being challenged by the weight and reps you're lifting, but "muscles feeling tired" isn't a good measure. If on your last set you feel like you couldn't have gotten more than one or at most 2 more reps, you're probably bout right. If on your last set you fail to lift all the reps you are trying, you are trying to do too much.
As Wendler noted in his article, people training to gain strength for sports activities shouldn't spend more time in the weight room than they have to. If you're getting bigger and stronger while spending a reasonable amount of time lifting, you're on the way to becoming a better rugby player.