What if one considers the following approach: Let e be a probability small enough that if I were to accept all bets offered to me with probability p<= e then the expected number of such bets that I win is less than one. The approach is to ignore any bet where p <=e.

This solve's Yvain's problem with wearing seatbelts or eating unhealthy for example. It also solves the problem that "sub-dividing" a risk no longer changes whether you ignore the risk.

On the margin I think its usually better to take less and/or easier courses. It is better to do very well on an easier schedule than do "ok" on a hard schedule. If you apply for a job or grad school everyone will look at your gpa. If a semester is too easy this is not a serious problem. You can always read an extra textbook or do some coding projects in your spare time. Next semester you can up the difficulty. If a semester turns out to be too hard and you do badly the penalties are real (though survivable).