Do not park them on grass???

What is the issue?

Killing the grass?��

The issue is moisture wicking up from the grass and into only that portion of the tire. You lose consistency in the tire and they weaken and blow eventually.

Parking over grass is fine, but it's best to put them on 2x12s or some folks even go as far as teflon sheets added to wood, placed down appropriately for tire track width.

Personally, I go with 2x12s on our trailers that sit outside.

If your boat sat for a while like a year plus would it be beneficial to jack the trailer up just slightly on jacks to keep pressure off the tires and avoid flat spots? And cover them for UV protection.. For example mine has been sitting for almost 3 years but will be going back and forth to a heated shop to work on over the winter. Also not talking a big boat either. 26' but 6k lbs tandem trailer.

Some folks do like to jack up by the axles or frame to take the weight off the tires and bearings. If it's easy to do, I'd say go for it. But don't bend over backwards.
When it comes time to put your project back into regular service; go ahead and invest in some new quality ST or LT tires.

During the winter layup I lift and support the frame of the trailer to take the weight off the tires AND axles.

I'm pouring a concrete pad where my boats sits so it would be ideal for me. And since I'll be buying new tires in the spring in going to protect my investment.