I have re read the OP clearly and most of the posts here have strayed away from the real issue mentioned in the OP. It is not about mixing in mono which I was referring to and it is certainly not about converting mono tracks into stereo either.
 
What he was really asking is it best to record stereo tracks onto two mono tracks instead of one stereo interleaved track. I briefly mentioned it in some of my posts as did Mike too. I think a good answer to this is that there are advantages to doing this such as the ability to pan the L and R sides of the stereo image in anyway you want which is good. It also allows for different processing on L and R sides should you need it.
 
There are some disadvantages to it as well. Takes up two tracks instead of one. Sending to a bus is a good thing to do but then you need an extra buss just for this purpose. It is possible to get the two halves of the stereo image out of sync.
 
But the good news is that a plugin such as Channel Tools from Cakewalk is very handy for dealing with these things and can achieve nearly everything that the double track concept can. You can totally pan the L and R sides of the image anywhere, you can polarity invert either side and you can even add precise amounts of delay to either side if you want to as well.  You can adjust levels of of L and R sides individually too.
 
So once upon a time two mono tracks would have been the way to go but now we have some pretty useful plugs to handle it. Even Pro Tools has gone interleaved as they had the Split File concept for years so they must think it is OK as well.

...we are just participating...Yes I agree and thanks for your input too Cassiano.  But it is interesting how the OP can go off track and end up on something else completely. I am guilty of it too.
 
I can also understand why the person who posted the OP may feel that their question has not even been answered. You have to read the OP actually very carefully to really get to the nitty gritty of what they were asking in the first place. And if they have not expressed themselves perfectly clearly it can be misinterpreted.
 
I think getting back to it though we can record stereo onto an interleaved track and still have lots of control after. I also like mixing and checking stereo mixes in mono on a small speaker at low volume. And yes there are some very interesting ways to convert mono tracks into a pseudo stereo effect. Going OT again (sorry!) I found the free SHEPPi spatial enhancer also very good for this sort of thing. It is all built into the plugin making it all much easier.

Mostly we listen to music in Mono  not matter what anyone says. In car if your steering wheel is on the right you mostly hear with the left ear and the same with left hand drive you hear with the right ear. In the house you are moving about so its mostly Mono as well. So mix in mono and when you switch  to stereo you will have a great sound. You will have to work harder to hear different instruments in mono and it's something I started to do and  getting better mix's I hope (lol) The pro's on here will probably give a  better description.