Agreed with kewms. Re-organize your Projects list during your Weekly Review.
Here's one way to look at it: One purpose of the Weekly Review is to have a time when you can calmly and completely prioritize your work. You can make smart decisions about all your projects, both current and future.
If, in the middle of your week, you toss more Projects onto your list, you're not in the same mode of thought as you are during your Weekly Review. You don't have all your Projects, Someday/Maybes, etc. out in front of you to determine what you really have time for. You can't analyze your workload as effectively as you can during your Weekly Review.
If a "new Project" comes along, toss it onto your Someday/Maybe list. You'll be able to put it on your Projects list in a few days during your next Weekly Review anyway, but you'll be able to completely analyze it against your other Projects.
Does that help and/or make sense?