Not at all, when you initially set-up the receiver you will either manully program it with your actual speaker configuration, or it will automatically detect the speaker configuration, and
you will not lose any program material. 
In fact, once the receiver knows you have a 5.1 set-up,
if/when it receives a 7.1 audio program, the receiver will automatically re-route and combine the surround backs' program material with the surround L/R speakers signal!
Damn these things are getting smart!
Having a 7.1 receiver
may have
additional advantages, depending on the make/model:
a) Able to power a 2nd zone in addition to the 5.1 main zone
or
b) Able to bi-amp your mains (a debatable proposition)
or
c) Power a pair of presence/height/width channels
.
In most cases, the
7.1 receivers are actually at the "heart-of-the-envelope" price wise and provide more value then the 5.1 receivers these days.
Cheers,
XEagleDriver
