You will need to put the rig somewhere where it will not get in your way. If you have the rig some distance from you, it may pay to use an extension lead for your microphone so that the lead is not tight across your steering wheel or selectors.
It can also depend on the likely use you will be putting the CB to - if you offroad with a bunch of people who always use just one channel, or go to offroad sites the do likewise, the chances are you're not actually going to need to touch the CB other than turn it on at the start, and turn it off when you're home.
If you're planning on speaking to many different people, who might be using different channels, or you just don't know who you're going to end up talking to and where, then you may want the CB a little closer to you.
Most CB's now have up and down channel change on the microphone, but if you're going to use this facility, it really helps to be able to see what channel you've ended up on!
Depending on your choice of location for the set, you may find the internal speaker (on the underside of the unit) is obscured, or too far away to be heard in a noisy vehicle, so an extension speaker facing towards you, or behind your head is a definite advantage.
(Pictured here is a Midland 48 Excel in a late model Range Rover Classic)