I can't speak for OS X but I know windows likes to try to take up the whole disk if you aren't careful when doing the partitioning. When I did dual
boot configurations with Linux and Windows I would always do the windows install first then let the Linux installer resize the partition to something
reasonable for the two operating systems to live side by side in relative peace.
If it is a desktop I will make a suggestion. Just add a second hard drive and use the bios to switch which one you want to boot from. It keeps
everything separate and safe from the other OS.
The above works for any OS combo. Boot managers are nice but if your not careful when adding the second OS, things might get screwed up.