It doesn't matter which order you install the operating systems. Check out the link below. It will walk you through the steps and how to use EasyBCD. I installed Windows 7 first and then XP on a second hard drive. Scroll down half on the page where it says Windows 7 installed first. It will show you how to set up the boot menu for both operating systems.http://www.sevenforums.com/tutorials/8057-dual-boot-installation-windows-7-xp.html
Yes, it does matter, but there are work-arounds. If you install the oldest OS first, newest last, the boot loader for the newer OS will automatically setup an option to select which OS to boot into.
Using a program such as EasyBCD gives the user control no matter which OS was installed first, and there are other work-arounds.
I've been using multi/dual boot systems since the mid 90's, including Win 98, NT, Win XP and several different Linux distros.