why not use 5.1 on a 7.1 receiver, there are usually settings on the receiver to select the speaker configuration, look in the owner's manual, download the full manual from the manufacturer's website.
The only problem with that is a lot of the surround dsp's will not work for you. Most of the dsp's in a 7.1 system are for 7.1 and are not usable if you only connect 5.1.
If you look in the owners manuall it will give you descriptions of the different dsp's and in what scenario they can be used. Within those descritions most manualls will have sections that are shades of grey or have small numbers in the upper right that correlate with a sub note. You will notice that the sub notes will say only available with 7.1
You can do it but you will be limiting the options of what Dsp's you will be able to use.