Are you sure?
I was under the impression that whatever minerals were collected, regardless of when they were collected, that the last save you make in ME2 , if you use that one, that ME3 will recognize what you did. I've never got 100 000 for each since it takes too long, but I was able to collect enough to get 25 war assets in ME3 and I collected the minerals after the suicide mission.
What I have discovered is that you're meant to get the 100 WA no matter when you gather the 100k resources, pre or post SM it doesn't matter, but the saves where I gathered the resources after the SM almost always bugged out, and didn't seem to count the resources I had gathered properly. I had to re-import them once or even twice to get the 100. Save's where I gathered the resources before the SM never bugged out, I always got the 100 WA's in my first import and didn't have to re-import the save again.
So yeah, you can gather the resources anytime, but I advise to do so before the SM to avoid the damn bug which stops the resources you gather after the SM to not be counted. That War Asset is so very bugged.