Wedding ceremony and reception are usually in 2 different places (requiring a drive between them), so no one would even really notice , unless its a very small wedding that you were not at the ceremony and only at the reception.
Just RSVP and say yes, make a note only reception if you want to, don't stress about it :0)
Having only gotten married about 12 months ago,I would have been happy for people who meant something to me to be invited in the first place, to have made the reception only if comitments made it that way, would be happy having them there, thats why I invited them in the first place.
Jason