Be careful when you say "downgrade". first of all, in most cases, Windows 8 does not include downgrade rights as part of its license. You can only "downgrade" Windows 8 Professional, and ONLY if the Professional version came with the machine when brand new. You can't downgrade Windows 8 "home" to Windows 7. Also, if you add Windows Pro as an upgrade, you can't downgrade it to Windows 7. The Pro version has to come with the PC when new.
Now, that being said, of course, if you own a legal copy of Windows 7, or buy one, you're certainly free to install it, a fresh install with its own new license, not a "downgrade".
Last, you should be careful. Very often, new Windows 8 PCs aren't compatible with Windows 7; no drivers are available, etc. Even if your Windows 8 Pro license allows the downgrade, that doesn't mean it will actually work. If you want to downgrade, check very carefully to see that the model you buy will be compatible with the downgrade. Check for driver availability, forum discussions, etc. The manufacturer's website and Google are your friends.