I think there aren't a lot of multichannel tracks with complex center speaker usage simply because most people do not have center channels that are of the same caliber as their fronts...
I actually personally have rerouted some elements OUT of the center channel to other speakers to provide a more stable or cohesive image, which is usually the goal, I think. For example, I do believe I changed the routing of the bass synth near the ending of "Optical Hunting" from "The Golden Bonana" on the SACD. On the SACD it appears in the front channels. On the DVD-A it appears in the center.
I used to treat all speakers equally and put whatever the hell I felt like in the center. For example, on "wise save" from toad's album "what an actor you are" I routed the kick drum exclusively to the center. Why? Why not?
Later on I realized that creating a cohesive sound image is kinda...important? LOL. So it's affected the way I use the center channel, because using the center appropriately can be very tricky in my experience. I assume this is what a lot of other 5.1 mixing engineers struggle with. Sometimes I'll put an element in the center and it might work great in theory but in practice it might skew the image inappropriately.
Initially, in Track 2 of what I'm currently working on, I had this deep, pounding bass exclusively in the center, but then realized that it was just like...making the center way too prominent and skewing the image weirdly and such. So, I 'redistributed' that element to the mains with a bit of bleed-in to the rears, and it worked a lot better. On Track 6, I have this deeper guitar that comes in the center, and the only way it really works is being in the center, because it needs that accentuation that a center can provide. So, with it being in the center, it stands out, but is still buried the amount I want it to be. One thing I've noticed I consistently flip-flop on is putting the snare of a drumset in the center...it seems that it's really dependent on the placements and elements of the rest of the song for whether it works or not.
My rules for center channel mixing NOW is to just do what makes sense? IDK, I don't really have a defined ruleset but I can feel when something won't work. Some tracks will have only reverb in the center. Other tracks will have synths, vocals, drums, etc. It's like with rear speakers. Sometimes the music only calls for lighter elements in the rears. Other times, put the damn drumset back there!
Although, I guess I've gotten slightly off-topic, as
@coolerking101 is looking for recommendations. I recall Bonnie 'Prince' Billy's "The Letting Go" having some heavy bass in the center along with vocals and some guitars...