These tags are all used similarly:
What should be done about them?
I feel that idioms and proverbs are clearly defined, and that we should impose their correct usage by retagging and comments. The tags expressions, phrase and phrases are more diffuse, so the two other tags should be preferred over these.
And just to make my understanding of idioms and proverbs explicit I'll quote www.thefreedictionary.com: an idiom is "a speech form [...] that is peculiar to itself grammatically or cannot be understood from the individual meanings of its elements", but a proverb is "a short pithy saying in frequent and widespread use that expresses a basic truth or practical precept."
Therefore "ins Fettnäpfchen treten" is an idiom, while "Morgenstund’ hat Gold im Mund" is a proverb.