Under Adobe's current one-version-back policy (introduced with CS6), yes, you will need to own CS6 in order to be eligible for CS7 at upgrade prices.
If you're currently on CS5 then you'll have to upgrade to CS6 before CS7 ships. If you're intending to leave it as late as possible (why?) then you'll have to be vigilant about when Adobe announces it.
Alternatively you can join the Creative Cloud and get all upgrades included in your membership for the same monthly fee.
No-one other than Adobe knows when CS7 will ship but there's plenty of speculation online.