I guess the answer depends on what you think you might need to use XP for. If there are some programs that you don't think would work under XP then you can probably just use the XP virtual machine that can be installed under Windows 7 and avoid dual boot altogether. But if you have older hardware for which no Vista or Win7 drivers are available then yes, you may need a dual boot.
Personally I've just installed Windows 7 all by itself and haven't had any problems running programs going back as far as Money 97, or with drivers going back as far as my Epson Perfection 2400 scanner (about 6 years old).