If the question and answer should be logged somewhere, then e-mail is one idea to use so that there is a record of this being asked at some specific point in time.
Does your e-mail software allow to mark e-mails as high priority? Do you know how often the person that would be responding would get to that e-mail? Some managers may be in meetings for most of a day and thus aren't likely to e-mail a response unless they take a laptop or smartphone into a meeting.
Something to consider is what kind of follow-up questions and answer are you looking for in the e-mail. If the e-mail is simply a, "Should we go ahead with the plan to implement feature X for this release?" where the answer is likely to be a yes or no without much wiggle room then e-mail can be useful. On the other hand, if there is likely to be a discussion about the question as the answer isn't going to be a simple binary one, then it may be worth calling or scheduling a meeting.
Something else to consider is who should know about this decision and how should it be communicated? If you are implementing a coding standard, then it may be worthwhile for all the developers to be in on the conversation while if the question is something that just you and the recipient would need to know then it may be better to address it in other fashions.
Last but not least, consider your audience and how much of an e-mail person is him or her. Some people prefer e-mail as a form of communication and others would want something else. There is also instant messages, text messaging, Skype, and a few other possibilities beyond face to face and phone calls that is worth noting.