At what point is eating left-overs or perishables not just daring but dumb? I am a college student who is sometimes a foodie, sometimes a garbage disposal. There is plenty of good food that some of my peers would turn their noses up at. But sometimes I wonder if this is not just disgusting, but actually bad for my health.

Food service standards are not what I am looking for here. Those are already clearly too stringent to me. Rather, I am looking for some rules of thumb about when I should not just cut or scrape the bad parts but trash the whole thing.
When in doubt, throw it out. It's cheaper than a hospital visit.

If you are wanting to save money, be proactive and find out the best way to store each food, and how long it can be stored.

Some things lose taste and texture as they age (even in the proper environment), while some things begin to grow dangerous kinds of bacteria or molds. For example, hard dry bread gets turned into french toast. Moldy bread gets thrown away.