Grub and /boot load before any drivers but saying this there are a few things with RAID.
Firstly when it comes to hardware RAID there are a huge amount of motherboards with RAID on-board. In most cases its what is termed as fakeraid.
Also with any hardware RAID there is one consideration that if your hardware fails you will probably have to source exactly the same hardware to ever read those disks again.
So unless you have a really top notch RAID adapter and quite a lot of spindles then I would say don't use low end hardware RAID and the difference with a few spindles doesn't have any advantage over software RAID.
The only RAID type that can be read by grub with /boot is software RAID1 a mirror or high end dedicated RAID device. So turn off your fake-raid if it is, also just a mirror might as well do it software wise.
Or on install have a small SSD or even USB for a 1GB /Boot which is big for /Boot.
Grub will install and update each spindle of the mirror. Even on RAID5 or RAID10 software arrays I keep a small partition at the end of each disk make it RAID1 and /boot and boot sectors are replicated.
Ubuntu isn't very clever when it automates the grub update and boot sectors. Practically always aims for SDA so try and get your BIOS settings right and your drive cabling.
I have a SSD Cache drive that has its own format and Ubuntu still uses that, I just unplug whilst I install.
Have a google about fakeraid and the disadvantages of hardware dependent disk reading. Software RAID like I say unless you are going for a high end, high spindle count.
https://help.ubuntu.com/community/FakeRaidHowtohttp://en.wikipedia.org/wiki/RAID