EQ is the most important part of mixing and mastering. Too much or too little can make or break a track.
I roll off everything around 30hz. Even my 808s. Most of my instruments around 100hz.
Depending on what the instruments are "saying" or the feel of the track I may add certain frequencies to add excitement and feel.
Every mix is different and everything has its place in a mix. I use eq and compression to make space.