The reputation requirement is low, but I don't meet the requirement yet. Should I propose it here or wait until I get the needed reputation?
The questions should be correctly tagged. So if you feel that there's no tag that would really pass to your question, don't wait, but write here what tag do you propose and why.
There are 2 possibilities.
1) Someone with high reputation will create tag and retag your question.
2) Someone will write explanation, why the tag should not be created, and which of the existing tags are more appropriate.
No matter which of those would happen, asking on beta is highly recommended.
Requesting that a new tag be created before the question has been asked seems a bit… backwards.
New tags are created out of necessity, based on suitability of the questions asked. So it's much better to simply to ask your question… and then either 'flag' it for moderator attention or discuss the question here… in meta.
That way we can look at an actual use case to see if the tag is warranted.
Go for it and just propose it (in another question) - if and when we get Moderators pro tempore, they'll be able to deal with the queue of requests for tags that we didn't add during the private beta period.
Another team that can deal with this are the Stack Exchange Community Managers, or even a few of the highest rep users (who also frequent meta). Meta is the correct place for this.