How bad is it to accept a competitive Fellowship, then reject it BEFORE April 15 My situation:

I applied to school A (R1 school, US News ranked, aligned research) and is nominated by POI for a competitive Fellowship. This fellowship covered tuition + stipend for a couple of years and comes from the school itself (not department). The fellowship offer is only through email, so no official signed letterhead yet. Since the Fellowship is competitive, POI urged me to accept it  "within the week"  otherwise it will go toward somebody else (one week after Fellowship decision). However, I:


Have reservation about the research group (haven't visited the place, only Skype called, their pressure for me to accept, outcome of graduates)
Still waiting on other schools, which are very promising (contacts with POI, recommended for admit) yet entirely unsure of fundings.


I know that according to the April 15 agreements, I can accept then reject the fellowship before the deadline without personal repercussion. However, I want to know:


How much am I screwing up the POI if I accept the fellowship then reject it later? I am afraid that an email confirmation then rejection would make his research lab and department looks bad to the school for losing potential spots.
With question #1 in mind, should I do it? The April 15 agreements seems to be set up for these situations, but is it selfish for me to jeopardize other people chances?
Let your ‘Yes’ be ‘Yes,’ and your ‘No,’ ‘No.’

It may be legal to accept and then reject the fellowship, but it does not make it right (in my opinion). Academia is still very much built on trust and goodwill. People who choose to scheme their way through loopholes in the regulations may benefit in a short term. But people who stand behind their word deserve ongoing respect and make for leaders of very strong academic teams. 

Don't do anything to people that you wound not enjoy done to yourself. If you are having second thoughts about this option, leave it and wait for something else which may be going your way.