I believe it was chorus/delay and reverb that got really bizarre at times if I messed with the mono/St interleave after recording with the respective effects.  That is at times as I trial/erred (much more the latter...lol) to see how it would effect the overall sound(effect and placement) in the mix....I have mainly stuck to mono going into most all the plugs I use in the chain.   Its a fun journey...just gotta get used to what works for ya out there and stick to a plan of sorts.  I'm about to really buckle down on the recording process and stick with a method of madness I feel will keep me on the right path.....lol...least that's the "plan".  

Cactus Music
That's right. If the source is mono, record it in mono.
Very few sources are stereo. Mostly outboard synths and guitar processors.
These are stereo and benefit from a stereo recording as that will capture the built in effects etc.
You can actually cause phasing issues with stereo recordings of mono sources.
The other benefit of mono tracks is the ability to pan them in the stereo field. It's a PITA to work with stereo tracks and panning.
And it's certainly easier to look at a mono wave form on the screen and see what's up.

I agree with the above - just want to point out that some drum kit pieces from sample-based drum libraries, such as with Battery 3, ARE stereo samples, where they used a top and a bottom mic on a snare or tom, for instance.  Just take a look at what you are using and set the interleave as needed - mono for mono, and stereo for stereo.
 
Bob Bone
 

A synth maybe a very stereo source even with NO processing involved.  Some synths may be all mono with some sort of chorus or stereo processing just added on at the end.  Sure this may be the case for some but there are some synths out there (hardware and software too) that have like 4 or 6 or in the case of a Kurzweil 32 layers per note. 
 
And these layers are all panned all over the place.  So even with no processing at the end of the chain the raw sound can be seriously stereo.  (Yamaha SY77 comes to mind. Switch off all the effects and nothing much changes)  Stereo tracks please in these cases.  Use your ears and listen. If you turn all the effects off and the source is still seriously stereo then stereo tracks are the go. 
 
Panning stereo tracks is not different or harder than mono. Use something like Channel Tools to control where L and R will end up.
 
 
 
 

Jeff Evans
 
Panning stereo tracks is not different or harder than mono. Use something like Channel Tools to control where L and R will end up.

Why not just use a pan knob Jeff ??

timidi

Jeff Evans
 
Panning stereo tracks is not different or harder than mono. Use something like Channel Tools to control where L and R will end up.

Why not just use a pan knob Jeff ??

Pan knob is just a balance knob for stereo tracks. If you want to actually pan, you need to use Channel Tools or similar. (= If you hard pan a piano stereo track with lows on the left and highs on the right channel, you can never get highs and lows on the same channel.The other channel just gets muted, the audio doesn't cross the middle to either direction)

Mono is all you really need for tracking individual instruments & vocals the vast majority of the time...other than drums I suppose - or if you're recording a solo grand piano performance or something along those lines. 

There are times where recording a solo performance in stereo is better and solo piano is one of them. Two mics on a solo piano and there are some good options for doing it. M/S, Blumlein, Near coincident and coincident or even an AB spaced pair under the lid. It is all going to sound much more interesting in the end. With only a solo performer.
 
A solo instrument such as say sax or clarinet etc could be handled with one microphone on a mono track a certain distance away but then I would back that up with a stereo pair as well capturing from a little further away etc.