Question for you: How are trees bad for water supply lines?

Really?
Weeping Willows will put their roots through cast iron to get to a water supply.

Other trees, depending up on their density, can crack sewer lines.

Haven't experienced that? Consider yourself lucky!

So, anyone? What would be considered a safe distance from my well?

"I know weeping willows and water lines of any type don't mix." A supply line is under pressure and only a developed hole-- which will leak- permits tree roots to enter. Tree roots cannot enter any piping system unless there is an entry hole.
Roots in a pipe means that the pipe was faulty before the root entered.
"What would be considered a safe distance from my well?"
So we go from questions about trees and piping to trees and wells-- different animals. A willow within 100 feet of a well is too close.
Older large wells of greater than 6" diameter need plenty of distance from any tree.
But my opinion has already been discounted.

Other trees, depending up on their density, can crack sewer lines.

My reading indicates that the cracks occur first and then the tree roots invade the sewer lines through the cracks (or loose joints) rather than the tree roots causing the cracks.

Hmmm.
Glad I asked. I've always understood the opposite.
This is one of the newer wells, with the tiny mushroom-type thing in the yard.
I actually still have a giant, concrete topped well about 100' away from that. The previous owner never filled it in. I have always dreamed of an irrigation system, but that's a dream.

So essentially, so far, there isn't an advisable distance for a water line of functioning condition?

Come to think of it, I'm not certain how I'd know if my well water line weren't in good condition. Soggy lawn? Constantly running well pump? Huh.

If you have a modern PVC line, and it remains undisturbed (no heavy concrete trucks backing over it to crack it, etc.) then you will have zero issues with cracks in the pipe. Older materials, like clay, can develop cracks over time and should be prioritized for relining or replacement depending on their age.

I wouldn't worry about planning trees anywhere around a modern sewer pipe, and in fact, I have a giant red maple growing almost on top of where the city decided to put the sewer connection in the yard. Other than the difficulty of accessing the spot to do a connection, there won't be an issue with having the pipe there at all.

Frankly, I doubt it. The big concrete well was put in in 1976, probably.

Sometime after that, they dug a deep water well. It's probably water they would have used before PVC. As a matter of fact, when we rebuilt, my DH changed the lead from the ground to into the house from a garden hose segment to some kind of pipe.

Good news about your maple, tho. How beautiful!