Hi! Rooting your phone is not anything that one just has to do. Your phone will work perfectly well for you without rooting it.
However, there are some advantages to rooting which is why many of us do it.
This is a good article on why to root:
Why you should root your Android
The Metro store would discourage you from rooting (if they knew about it) just because it does void your warranty. I don't care about this as I plan to buy the new Huawei IDEOS X5 or X6 as soon as Cricket gets them in mid-2011.
But after rooting the phone, it improves the performance and makes it run more smoothly and a little faster. The Ascend tends to lag, due to a lack of HP.
When you can remove unwanted apps (commonly referred to as Bloatware) it frees up your phone to run faster.
You can't remove stock apps (those that came with your phone) unless it has been rooted.
If you have any doubts I wouldn't do it until I had read more and felt more comfortable with performing these actions. But if you do and you have problems, there are those of us here who will be happy to help you.
MetroPCS will not be able to help you.
One thing you can do without rooting (if you haven't doen this already) is to get a new launcher from The Market. I recommend LauncherPro Plus. It will make you think you have a new phone!
As for the apps you have downloaded. It won't do anything to them. If you are concerned you can download Titanium Backup first and back them up.
I hope this helps! Good luck whatever your decision!