In retail, the customer is always right. In writing, the muse (or voices in your head or inspiration or whatever you want to call it) is always right. To put it another way — when you muse says write something, WRITE IT.
Don’t argue with your muse. Seriously, you’re going to lose. And if you argue with your muse long enough, you’re going to get writer’s block. Your muse is always right. Just do what your muse says and your story will be happier.
If your muse decides to cook up an action scene that isn’t supposed to show up until the end of the book and you’re on chapter one, WRITE IT ANYWAY. Get it down and out of the way. Don’t put off inspiration because of some silly notion of the “right way” to write. The right way to write is to WRITE.
If your muse tosses a plot bunny at you for a book you aren’t even working on, open up a text file and jot down enough notes (even write a few scenes) so you can go back to it and know where the story was headed once you finish the current WIP. Don’t depend on your memory. The very things you say you won’t forget (the idea is too good. There’s no way you’ll forget) are usually the first things you do forget.
Create an ideas folder on your computer and stick all the text files of your notes in there. That makes it easier to find them later.
As with all things on your computer, MAKE BACKUPS. The same thing that can eat your manuscript will eat your notes too. Or you could always keep handwritten notebooks. Up to you. But remember to OBEY THE MUSE.