I think it would be valuable if you could be specific about what you need here. The basics of the question get covered in a lot of books on the topic.

So what is your question?

Hi manodipan,

Placing the switches at the front end might create the problems you mentioned if the sensed signal is coming from a very low impedance source. However, if the source impedance is comparable or higher than the one from your switch, then it might not be that problematic.
In other words if offset and noise are critical placing them at the input might not be a good idea.
The advantage of placing them there is that the signal swing is negligible and nonlinearity can be neglected (which might also hurt you).

At the output you can forget about the offset and noise contribution from the mux switches (assuming your front end gain is enough to consider so), but you might end up having some linearity problems due to large signal swings.

So I would make the choice based on the parameter that the switches compromise that is less important for your application.

Regards
Tosei