if you are mounting it inside your car like under your seat or something I wouldn't. But if it is in the engine bay... I would strongly recommend it. Most people have problems with the MSD's when it is close to heat for a long time or is constantly vibrating.
In this picture you can see where mine is mounted(the red box there behind the headlight). I have had it solid mounted for the last 2 years with no problems. Of course I have only put about 4,000 miles on the car in that time, but nonetheless it has gave me no problems.
not an MSD, but i DO have my Crane HI6 mounted on the passenger side of the trans tunnel. and yes if you are mounting your ignition box on a solid surface(which i dont recommend) you need to use the rubber isolator mounts.
The two things that are the hardest on electrical components are - you guessed it - heat and vibration. Anything you can do to remedy those two things will improve the reliability of the ignition system.