I have the same issue. My timetable will have clashes if I don't get my first preference for each class. Will Allocate+ fix this for me? Or will I have to manually change it when I receive it?
Allocate+ will attempt to allocate you to your other preferences from 2nd down if you don't get your first preference. The only time which you'll need to manually change it during the first in first serve mode is when Allocate+ isn't able to allocate you to any of your listed preferences (due to a number of reasons - clashes, class at capacity, just to name a few..). By the time the sorting period is over, your timetable should be complete.
I would wait though - because even if you don't get your 1st preference or the one you actually want, ALOT of people change subjects, drop out or take leave for whatever reason which frees up spots in certain times. Only after census date is it really locked in.
If it turns out that you really wanted a certain time but didn't get into it, you can always show up to that class and ask if you can join. I'm sure if there's a spot available, they'll let you sit in.
Your preferences are
independent of each other, meaning you will ONLY get one of them. Your timetable in the end should be fully complete with all your class times locked in. The whole idea of Allocate+ is to assign you a clash free timetable, whilst trying to give you your highest preference. It's a very complex algorithm to understand and get your head around.
But the overall result
should be a complete timetable with no clashes (which may mean you might not be happy with some of your allocated preferences!). You will be very lucky if you get your first preference for every single subject.