At what point is eating left-overs or perishables not just daring but dumb?
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.