Probably several answers; I'll begin with a popular one. You should develop the load under the type(s) of condition(s) where it will most likely be used; i.e. weather, shooting position, firing cadence, etc. Simply, why do it any different from that?
Another: I'd use the bench/bag, since it eliminates some of the additional elements , i.e. bipod preloading, etc., that can complicate the load development process.
Overall, whatever suppport you use, take care to eliminate as many added complexities as you can from the process. Make the load the key, and if possible only, variable.
Greg