JC,
Most are positioned at the chart table area. One thing to remember as Rodger suggested, if not using a handheld, have the ability to operate it from the cockpit.
Most of the time using it, your coming to a bridge, in a channel maybe with boat traffic. These are times when you need to be at the wheel with mic close by. I hang my mic over top of the companion way hatch opening (STBD side) for quick access.
There are different setups, such as longer mic chords, cockpit speakers etc.
It is advisable to get a DSC ready VHF. Hopfully you have a GPS and can fish a NMEA 0183 04 or NMEA 2000 wire to your VHF. You can go online and register an MMSI# for your boat. Your DSC VHF will be what I call "a poorman's EPIRB."
Check the advantages out for this, and I believe signing up is free from Boat US. There is a form for all info on your boat. If an emergency pops up, hit the button and your current position and pertinent info is immediatly transmitted for help.
CR