Normally springs should only be replaced if they are not strong enough to keep the load stable or if they are cracked. If your trailer bounces a lot going down the road you should check to see if the springs bottom out when you push down on the sides of the boat.
Is it a home built trailer or brand name made for the boat you have on it?
If the trailer is not the one that came with the boat from the dealer what is the weight of the boat empty?
What Max Weight are the trailer & axle/axles rated to carry 2 wheel or 4 wheels? There should be a small metal placard near the tongue with the Max load rating, usually on the left side as you face forward.
Have you added a larger gas tank, shad tank/ live well or extra batteries to the boat?