Purest panning law

I'm not sure what you mean by 'purest' the only reason noramlly you'd need to worry about what panning law that's in place is if the project is going to be completed somewhere where the pan law is different to the one you are using.
 
Basically if you have two mono tracks at full-scale when they are summed together you will end up with +6 dbFS so if you are wanting two hard panned tracks to sum at 0 dbFS you'd use -3db constant taper to attenuate the level when you move the pan pots to the center.
 
Alternatively you just use whatever pan law you are set up with (default) and mix accordingly the only time doing that will ever sound wrong or lead to 'overs' if you change pan law mid project or use automation on the pan positiions and haven't taken into account the amount of gain change happening due to the summing taking place at the pan law you are set at.
 
IOW don't sweat it.

It makes sense to mix with any pan law setting you want providing you don't change pan law midway through the project otherwise you'll be needing to adjust all your relative levels again after the change, because you'll be taking into account what pan law is in play whilst you mix because it will sound right and all you change with the pan law is the relative gain structure between L/R and Center.
 
Set it and forget it unless somebody wants to share work on a project with you and has a different pan law in play or you do a fair bit of automation involving panning and your current settings are not working for you with that.
 
Wiki puts it like this.
 
The idea is that when one directs signals left and right with the pan pot, the perceived loudness will stay the same, regardless of latitude.
 
http://en.wikipedia.org/wiki/Pan_law
 
So yes the -3db setting is a common one, but if you are collaborating with other Sonar users you might want to check what pan law they are using so your mixes interpret in the same way as theirs.
 
As for mono compatibility that's another different subject but for your simple example you can reduce the width of your X/Y signals by panning anything less than 100% hard left and right but of course too much and your carefully recorded stereo image will start to become more or less central again.