What if assistant teaching is being used as a verb? Hyphenated or not? As in:
I have been assisant teaching with my grandmother for four years.
In that case, "assistant-teaching" would be hyphenated, yes.
http://grammar.ccc.commnet.edu/grammar/marks/hyphen.htm
Here is an excellent section on the uses of hyphens. Scroll down and note the part called Hyphens have other uses, especially "a."
When using the phrase "assistant teaching" as a verb, it is not necessary to hyphenate the words. You can simply write it as "I have been assistant teaching with my grandmother for four years."
However, if you want to use a hyphenated form of the phrase, you can use "assistant-teaching" as a compound adjective to modify a noun. For example, "I have been engaged in assistant-teaching activities with my grandmother for four years."
In general, hyphens are used to join words together when they are being used as a compound modifier, helping to clarify the meaning of the phrase. In the case of "assistant teaching," since it is being used as a verb, there is no need for a hyphen.