What do you like as a height for a workbench, and what do you plan on welding on it (mostly?)
I've built my workbenches to above belt-buckle high on me, but I'm a knuckle-dragger. Most people like them to come somewhere between the belt buckle and their navel. This will allow you room to work on larger projects, not have to bend over to reach smaller projects, you can use power tools that are mounted on the bench, and it's all fairly easy.
If you're going to work on taller stuff, make the bench proportionally shorter so you've not got your hands stuck up in your face. If you're consistently going to work on smaller stuff, you make the bench a little taller - so you can see it and reach it.
Honestly, the only one who can really decide how tall a bench needs to be is you. You can get input, but only you are going to be able to decide if you can work comfortably there.
As an ergonomicist (I've designed office furniture for people, workstations, and I've got quite a few happy clients,) I know this to be so. You have the final say - so make sure you're in a position to say it.
If you get adjustable feets, make sure of two things: 1) They're rated for the sort of load you're going to put on them, and 2) you're only going to get an inch to an inch and a half of adjustment out of them anyhow.