I have an idea for an answer to a question that I think might be helpful, but the question is outside of my area of expertise, so my answer might be more harmful than helpful.
How do I decide to post my answer? Should I put the balance on trusting the community to upvote or downvote my answer based on whether it's a good idea? Or should I put the balance on letting people who are more experienced with the question's topic provide answers they are confident are high-quality?
There is no clear-cut "do this or that" anwer to your question. It depends on the specific question, your idea to answer it, your degree of uncertainty about the idea, if the idea can be scetched in two sentences in a comment (or not), and how much you care for your score.
Some general recommendations:
if you are unsure about the correctness of something you are going to write on this site (and it does not matter if it is a comment or an answer), include the assumptions you made, or which parts you are not sure about, in your writing. So readers are warned for not taking all what you wrote for granted.
If you think you can contribute something of value to a question even though you are not an expert, you could write an answer starting with "I am not an expert on [this topic], but ...", . I did this from time to time in the past, seldom getting more than one or two points for such an answer, but sometimes my answer was received well anyway. Here is one example for an answer where I got more than two points in return.
try not to write something which is blatantly wrong. If you are unsure, do some research, use Google first and inform yourself at least to the degree you can be sure not to write any nonsense. Or ask the community, maybe by using a comment below the question, or by asking a specific question here on meta, where you reference the question you want to answer and give some information what your idea is.
don't take it personal if you get downvotes or comments for an answer the community does not like. There is nothing so bad you cannot at least learn something from it, and if you get one or two upvotes for an answer, that will balance all the downvotes you can expect for the same answer. And if someone tells you why your answer has severe issues, you can always delete it, improve it and then undelete it again, if you like.
don't be afraid of having someone else writing an answer "stealing your idea" because you scetched it first in a comment. This is a community site, our primary common goal should be to collect good questions and answers, gaining points should only be secondary.