Is it true you should wee on lettuce?

Yes and no!

Yes because lettuce love nitrogen as it helps them grow nice big leafy leaves. Urea is nitrogen rich and therefore yes weeing on your lettuce gives them nitrogen which helps them to grow.
But ultimately no you shouldn't. It's always best to keep FRESH human and animal excrement away from things we are going to eat. Coffee grinds, grass clipping soaked in water or well composted herbivore animal manure is much better.

** NB the manure that we use on our gardens is from herbivores. I don't understand the ins and outs of it, but omnivores and carnivores can have bacteria in their excrement. Therefore only herbivore manure is used in gardening.