Of course you have to be sitting in the sweet spot and you must not be using a discrete mutichannel source
Most DSP's handle even discrete data and will allocate the information to the speakers as necessary.
Obviously if you don't have a DSP and you are competely missing Center information then it will sound terrible. The same applies if you have a 0.1 channel and are not using a sub or do not have the ability to program the DSP to send 0.1 channel to your main speakers.
Missing channels DO need to be added back in to whatever you have in terms of speakers in order to mantain balance of the sound field (this is important if you have LESS speakers than discrete channels and is another reason why surround gest a bad rap from stereo people, as they don't do it justice by making sure they are using ALL the available information)