I applied to several Grad schools and recently got a letter of acceptance from one of them. Should I inform the other schools about being accepted? Will it improve my chances with them or just create antagonism?
You should definitely tell any school you no longer want to attend as this will save them a little time. Any school that you would rather attend, you should contact and let them know when you need to make a decision by. Not all grad schools do admissions in an atomic manner. If a higher choice school wants you, they may be able to hurry their decision so as not to lose you.
I can't imagine a situation where letting a program know about an acceptance to another program improves your chances of admission. To me, if I was on an admissions committee for school X and got word that you got accepted to school Y, all that would tell me is that you met a (largely unknown to me) set of criteria that school Y uses, which might share all, none, or some of the criteria that I use for admitting students to school X. By the same token, I don't think it will create too much antagonism.
On a personal note, I found that communicating with different schools AFTER I received a decision helped me. For example, I was accepted to programs both with or without funding. By letting the programs that initially didn't fund me know that I was accepted to a (rival) program with funding, I was able to turn one of my offers from non-funded to funded.
As others have said, once you know that you won't be attending a program, it is common courtesy to let that program know, particularly if you have an offer with funding.
In general, you shouldn't notify before you have heard from all schools unless the acceptance letter from School A has an expiration date and/or you will go to School A regardless of the other offers.
The risk is if you tell School B that you got into A before they make their decision:
If B feels A is stronger in your area, they may not want to risk an admissions slot and roll down to the next candidates.
If A feels as if it is superior to B, it may wonder why you're telling them this info. Are you seriously overrated in B?
If you have offers in hand from both, it's certainly in your best interest to let both know. You may be able to get A and B to bid against each other for you.
Admissions slots are very precious to departments. We don't want to waste then on candidates that won't come, and once we've made an offer we want to ensure the candidate will come.
That being said, once you've made up your mind, please notify everyone so that we can free up your slot for someone else lower down on the admissions list.