Do enough people know IPA that I can use it instead of phonetic spelling? Or should I use both? I tend to use phonetic spelling just because it seems easy to read, but IPA would definitely be better if people would understand it.
IPA is a great tool for linguists. Most people don't know it, however, especially not readily, and a lot of people don't even have the fonts installed on their computers, so all they will see is a bunch of little boxes. I'd avoid it just on the principle of reaching more people with standard phonetic pronunciation.
It is up to you. You can use either of them (or both). If the question uses IPA or the phonetic spelling, do the same in the answer. Take cues from the nature of the question or on the asker.