If you don't go to a scale, how are you going to know what the tongue weight is so that you can select the correct spring bar rating or if you are exceeding the TV payload capacity. Our factory dry TW is 540 lbs but the scale showed it was 950 lbs. Had pre-purchased a WDH and ended up having to buy heavier bars.
Another reason to go to a scale is that some TTs can have a low CCC compared to other makes/models of comparable length and you could end up exceeding the GVWR if just guessing weights. Overloading above the GVWR will void the warranty on the frame and could overload the tires (leading to a blowout). Doesn't help that some TT manufacturers install axles and tires with a low reserve load capacity.
I would say always weigh it in the very beginning and then maybe a few years later, esp. if you've done some mods & upgrades. I weighed our brand new TT on the way home from the dealer, after some mods & upgrades were done and then after it was fully loaded for camping and have some VERY useful info.
Weighing from side to side can help too since one side can weigh more due to a slide out(s) and what's in it and can overload tires on one side.