I rarely weigh before I tow, its time consuming, but I have a baseline of information and my vehicles have big margins in terms of weight. I don't try to fit the last ounce on my truck or beyond. When I'm going to the middle of no-where, I measure to make sure I don't contribute to an unexpected break down, and weight in.
I'd say most people don't weigh at all, one of my customers is a trailer manufacturer. They say their customers attitudes are if it fits, it ships. Judging by the number of F-150's I've seen with trailers loaded with stone and the trailer wheels tilted in, or flat..... I'd say its true. Lol, I've seen people in toyota tundra's towing 40ft 5th wheels at 85mph on i-10 (recently). I guess a tundra can tow a space shuttle, it can tow any 5th wheel... with airbags of course.... Anyway, the chances when you're overloaded of something breaking go way up, and if the nearest ram dealer is 150miles one way, then you don't want a break down. Or a ticket, but again its not something really enforced, I've never seen any personal vehicle pulled over for unsecured loads or being overweight. Commercial vehicles occasionally, and how many dump trucks don't pull their cover over the bed when loaded with gravel. My windshield knows its rare...
What I'm saying is what you should do, and what people actually do aren't always the same thing. The only way to know you're towing with the correct weights and distribution of weights is to weigh everything. Its what any individual should do to ensure the best towing experience. Work with actual measured data instead of guesses. So you don't end up posting about how your shocks only lasted 20K miles, or those damn "china bomb" tires blew out. So much of these problems are preventable with not-overloading, towing level, proper tire inflation, verifying weights, and etc.