If you are using it as a verb, you would never hyphenate. "She will warm up as soon as she reaches the cabin." When used in other ways, you might find it hyphenated or as a compound word, warmup. When used with the sense of warm-up exercises, I would usually use the hyphenated form.
does write up need a hyphen
It does not need to have a hyphen! :)
Written as: dress up or dress-up depending on when and where you use it!
No. Just looked it up in a printed dictionary. In fact, no work with the 'multi' prefix uses a hyphen. For example, multilateral
No
It's not a word in the dictionary, so yes, use the hyphen.
I do not believe that multitasking is supposed to have a hyphen.
You typically need only capitalize after sentence-ending punctuation, which a hyphen is not.
stretching
No. As back up, it is not included in the AP Style Guide, which means no hyphen is required. If you're using it as a verb (as in, "she will hand deliver the message), there is no need for a hyphen. But if it is used as a compound modifier (which means as an adjective), you WOULD hyphenate it: "she is taking the hand-delivered envelope." David Price, Writer
no
No