I learned this painful lesson the hard way several times
When using PM 8.0, you MUST MUST MUST MUST MUST decide on your partition scheme and sizes and use PM to create these. It doesn't matter if you change to reiser from ext3 while installing, but the sizes of the partitions and their locations on the disk must be decided from within PM 8. Otherwise, if you open PM 8 from within windows on a dual boot system, PM 8 will AUTOMATICALLY CHANGE what it considers to be a disk geometry error (some educated? guessing here) and will leave you with unbootable linux - even with a boot disk (read kernel panic). I was surprised at the automatic part because PM 7 told me it thought there were errors but asked first before hosing my system
I'll use my system as an example.
40 gig /dev/hda
hda1 win98
hda2 extended
hda5 data vfat
hda6 winswap fat
hda7 linux swap
hda8 /boot ext2 (RH8 /boot with grub)
hda9 /home ext3 (shared home directory)
hda10 / ext3 (RH8 root directory)
hda11 / ext3 (SuSE 8 root directory)
hda12 / ext3 (Slack 8.1 root directory)
This entire partition structure was created from within PM 8 which recognizes ext3 but not reiser. In other words, I decided on the sizes and locations from within PM 8 and formated them either ext2 or ext3. If during installation of SuSE i wanted to use reiser instead for / , that would be OK so long as I did not change the sizes or locations of the partitions outside of PM 8. Doing it this way, I can review partition information from win98 without hosing the system. (why i would want to do this I can't say

, I just like it when thing work the way they should) )
(note: PM8 does not read reiser so you will get incorrect partition type and used space when PM8 tries to read this but "I dont think" it will wreck things)
Leaving just a large unallocated space for linux will cause problems as will creating a large formated linux partition that you break-up later in disk druid. Don't get me wrong, you can create a usable system this way. Just make sure you delete PM8 after your done to ensure you never accidentally open it an make your linux partitions unbootable.