I go 225 pounds, not 300, and my Flows, built up with Dave Thomas SpeedDream cut spokes and tensioned / built to his specs, have been rock solid. I also know JeffJ built his wheels and we also tensioned them with the same spokes /specs, and he goes 275. I don't know if he's ever had to retension or true since being built either.
When a wheel is built right and tensioned properly, it should go a long, long time and a lot of miles, provided you don't abuse it or exceed it's limits, before ever needing a touch up true.
My guess is that at 300 pounds, you will definitely want to go with more than a 530 gram rim like a Flow, if you ride hard and fast. You just flat out need more material there in the rim to handle the load. Maybe a Salsa Gordo or a Chris Holmes rim?
Another point worth mentioning.... do not chintz out on your hubs and axles at your weight/size, big guys destroy hubs with scary results when things fail and crack/explode/go boom.
I'm putting Dave Thomas out there as another option for you as a wheel builder to consider. Many will vouch for his wheel builds as top notch.