mono.
the more mono the sound, the more likely it will be present in the mix.
typically you reserve things more stereo to be in the back ground. This is including harder pans and volume adjustments. Also depending on the buss setting (pre and post) and buss effect (reverb) will also depend on it's width and location in the mix.
It's good practice to slap on a width control aspect and make things more mono in the mix that are more important. Things with less importance can have less mono, or more stereo. If you're using buss effects correctly, this will allow you to adjust the amount of effect (pre and post send inputs in conjunction with channel volume) being projected, then you will ultimately create the buss effect that is placed well around the original sound source.
By doing this technique, you will achieve a better stereo image over all, because you controlled individual aspects that are being summed as one unit.
This will apply to drums and any component you want to use.
Interesting perspective in regards to things being more present in mono. I'm not sure whether I understand though because I always felt that when I gave an instruments stereo it made it louder in the mix versus being in mono. Then again I do notice that whenever it came to mixing lead guitar melodies or vocal melodies they were always kept in mono with the exception of the reverb/delay.
It's not a loudness thing, it's a perception thing.
it's not really an opinion, i've researched this to be a pretty fool proof and consistent method way of doing things.
It's only mono in the stereo sense of things, but not mono in the panning sense of things. I could give two shits about how you pan stuff, what i do care about is how you control the stereo aspect BEFORE you pan the stuff. and this control before you do any panning (or after but as long as you do it), is where you'll ultimately balance effect and signal as i've stated above with the bussing effect (more on that later if you want to pm me or start a new topic).
Quick google search, if you haven't done so.
GearSlutz^highly recommend you browse this
This blog^more about recording, but if you understand this you can better understand creating your own samples and the samples you'll be analysis for use
The moral of the story that you should of got was that, you need to control the image of the individual components in order to best deliver the clarity of the mix given the parameters and the desired results of the parameters.
And this is not panning, that's location of the space you're in. this is about, how big of an area you want the original signal to cover with in that location.