All of the take-home interview questions I've done have been given after a phone screen has already been done. I would think that's the only way to do it - it's going to take some time for a company to look over what you submitted as well. I don't think I'd bother doing one without that.

FWIW, I've done a total of 1 for each of the last 2 job searches I've been on, and ended up getting an offer out of each one, so it seems perfectly worthwhile to me to do them.