With a V3, it's pretty easy to set the the levels exactly the same, even though one is running low. That's what I do. Then when you mix in post and you decide to create a mid/side 40/60 mix, you know that's what it is. Personally, I think knowing this is more important that trying to squeak every last bit of S/n ratio out of a very quiet set of mics/pre... at least as a learning tool.
When I'm running knobs which don't have that degree of accuracy, I do what these other guys suggested, which probably means I am recording the side about 6db boosted up front, and then "mix to taste" afterward, with no idea what the actual mix is...
As an extreme example... imagine running M/S at an outdoor festival 200' from the stacks. Basically, everything is coming into the mid and nothing is coming into the side. If you crank the side gain up by 20db you can get some sound, but of course, all you are getting is off axis coloration. If you mix that 50/50 it will sound like crap. Recorded as I mentioned in the first paragraph, with the side contributing almost nothing, will sound very mono, but it will sound 'right'.