If you are doing a 5.1 even with no center channel it will make a big difference. It is crucial to do rear speakers in a Dolby digital surrond system, with most DVD players like Kenwood DNX9980HD having a built in 5.1 processor it makes sence to do rear fill speakers. when you plan on listening to 2 channel stereo you can always shut them off. However it is nice to have a rear fill.
If you were on a budget and you did not care about 5.1 then it is a waste and I woyld rather see you put the money in front staging. in a nother words I would not scarifice front stage to do rear.
On a stricktly 2 channel set up with a sub only, th echoice is yours.
As for speakers for that location. You may want to consider the
High energy 3" inch Midrange speakers, they are very shallow and they sound better then most 4" speakers out there, and some lower end 5.25" speakers.
If you wanted add a tweeter to that you can use a 2 way crossover and a set of Tweeters as well.