In general, undergrad experiences are probably not that useful on ERAS for a couple of reasons: 1) most people want to know "what have you done for me LATELY?" and 2) if you did great stuff in undergrad, someone somewhere will ask you why you haven't been doing it since. Finally, I can't remember but it might actually say somewhere in ERAS not to do it.

At any rate, if you have good answers for the above questions and ERAS allows it, then I guess you could put them in, but I'm not sure if it would actually help your application from the people who'll decide whether or not to interview you (med school performance/activities and tests tend to help you here). Now, AT YOUR INTERVIEW (or maybe tucked somewhere into your personal statement) your previous experiences might make for good conversation topics and help you there.

I put down experiences from before med school. The 2009 NRMP stats showed that applicants had, on average, around 2-3 work experiences. Are you meaning to tell me that the average student had 2-3 jobs during med school? Other than the occasional bartender, I didn't know anyone who has had a job during med school.

As for research, the Dean of Student Affairs at my school said to not bother listing undergrad research. There were classmates of mine from years before who had killer undergrad research, though, and little to no med school research. You better believe they listed that research. Based on how well they matched, with all things considered, I'm sure it helped, too.

ERAS instructs that teaching experiences be listed as Work, which I think many med students participate in during medical school or other gradaute training. For example, most of my Work experiences listed are from TAing and research mentoring I did during med and grad school.

In general, I think you need to use some common sense. Do you think adding an undergrad experience is relevant and something you are proud of or are you simply trying to fill up space?

I feel you about the CV padding. If anything, I left off a lot of things I see my classmates including just because I think no one could care less about them. I think for those who didn't go straight to college or medical school, listing those experiences during those times is relevant, though.