I was looking at options on a new boat and trailer this weekend, and one was to get shocks on the trailer.  I thought that might be a good thing to protect the boat and trailer on some less than perfect roads I would be using.  

A salesman told me that he personally thought that I didn't need them really.

What do you guys think?

It's really not necesary, but you would notice the difference in towing if you had them.  Everything in and on the boat will ride much better than with the constant bouncing.  Lots of travel trailers are offering them now.

It depends on the boat, weight and balance of the trailer... I don't have them on mine (19' champion w/ 175hp) and don't think I need them.  I have pulled other people's boats that I think SHOULD have had them.  If you are buying a new boat, you will lake test it first so pay attention to the feel and watch in your mirrors while on the way to the lake... other than that it's a guess or someone elses opinion.  

I feel a big difference with a full tank of gas (40 gallons behind the axle) in terms of bounce... it bounces less w/ a full tank on those crap roads leading to most ramps.

Just remember that will be one other maintenace item on your list, and expense down the road so don't just opt for it because you can...