When I have a "waiting for", I decide when I'll next need to take action on it (e.g. when I'll need to remind someone), and put a note in my Tickle file. I don't have a separate "Waiting for" list.

Reasons to have a separate list could include:

-- The "waiting for's" may be reminders, not actions. That is, you may want to stay aware
of them so that, for example, if the person hands you the results you don't stare
at them blankly and say "What? Oh, yeah, that." It works better to look at a
lot of reminders one after another than to mix reminders with things requiring
immediate action, because the frame of mind you need to be in is different.

-- Looking at the "waiting for's" regularly may help you avoid piling too much
work onto one person

-- When you receive the result, if you want to remove the item from your system
it may be easier to find in a "waiting for" file than in some unknown date in
your Tickle file. (I just leave it in the Tickle file and remove it when I come
to it; I rely on myself to remember that the thing is already done.)