Two items to consider that are not completely covered are:
Sharpness due to shutter speed and lighting control.

Follow David's advice and test w/ a fair amount of ambient light and test at different speeds with corresponding aperture adjustments and you will notice your DOF will change. As the exposure gets longer the flash will be less prominent -- this may or may not be a good thing, but it is an important thing to consider.

Use this technique on people and you will loose sharpness from their inadvertent movement as the exposures get longer.

Even if you are using a tripod, a passing car, shuffling your feet (or if you live in house like mine) passing train or strong breeze can cause camera shake.

If you set your lights up and test in total darkness the spread of light with the modeling lights and shoot the subject with only the strobes you can have more complete control of the lighting.

On the flip side you can use the strobes as fills for ambient light and shoot as the conditions warrant.

I'm not sure that knowing the 'best' shutter speed is as important as knowing how to achieving the desired effects within the known usable extremes ( between 1/300 for a strobe like Nick's and somewhere around 1/8 -1/30 when shooting people at an aperture that will give you the best DOF).

FWIW I know a fashion and product photographer that swears that the faster the shutter speed the sharper the image. He uses MF equipment with in shutter lenses (max speed would be ~1/500 or maybe 1/1000 -- no good for the Nick's Metz). I am not sure what he knows to justify his argument, but there you have it - my 2c and an unknown somebody.