Adam,
A bonus question offered to all students is probabbly a good way to assess knowledge beyond what is required. For me, if material is required learning, I want assess to what degree the students have learned, so I personnaly do not give extra credit. However, an essay type question on a more difficult topic when the test is mutiple choice (either electronic or scan-tron) has offered a way to earn 2-5 "bonus points". Out of of 125-150 points, that doesn't help a lot. but does reward those who attempt it, and in the big final grade, does not distort the grading system, such as a whole bunch of points given as extra credit would do. That said, each institution and instructor needs to decide what works best for their class. Many institutions have blanket policies that prohibit awarding EC (as it it diminishes the accuracy of grading and contributes to artificial grade inflation, something frowned on by career schools).
Barry Westling