Another method to consider (for overall tempo variation, not before/after the beat per track as mentioned above) is to actually perform the piece (no click) on one instrument and get that tempo variation (in general).
A massive pitfall with a DAW is "snap to grid" and tempo maps have traditionally been static and hard to manage. If you work ITB exclusively, realize that can be a massive liability.
Static is boring. Dynamics come from tempo variation, volume dynamics (why squishing everything is again boring), frequency content (fading components in and out), general motion in the sound stage, and the vocal (not only inflection changes, but also the words themselves).
A litmus test I do often is play back new tracks by listening to the first couple bars, skip to 1/3 in for a few, then a couple jumps into the last 2/3 area. If those snippets all sound extremely similar, it is a good indicator of the rest.