It's all a matter of preference...
The stave doesn't care whether you are flexing it by hand or by string... as long as you don't over bend it.
You can get it ready to put on the tiller by floor tiller, careful measurement, feel, luck or whatever... it doesn't make a jot of difference!
At some point you have to flex the bow a bit to see if it is bending evenly.... do it with a long string or on the floor... it doesn't matter in the slightest.
Personally I do very little floor tillering as I think it's easier to see it looking square on with it on the tiller.
The advantage with floor tillering is it is immediate, you don't have to mess with strings... but that's it.
If anyone disagrees with this please send your objections to Mr J W Halverson
Del