Etiquette for proper usage of Greek letters and other notation

Question

I've progressed to the "output" point in my mathematics career and have run into a slightly embarrassing problem while writing a paper. Clearly, certain Greek letters are suitable for some situations ($\epsilon$ and $\delta$ for small numbers, etc.) but is there any comprehensive etiquette for naming that I should be aware of? It seems like arbitrarily assigning sets and functions Greek letters could be a bit confusing to the reader.