EDIT: Starting Feb 1, 2014, Apple will no longer accept pre-iOS7 apps for submission to App Store. So while this technique still works, it will not be useful for most readers.
Yes, this is fine. I still build with iOS 4.3 for one project (it's been awhile since we updated; but they still accepted it after iOS 6 came out), and I currently build 10.5 apps with Xcode 5.
See How to point Xcode to an old SDK so it can be used as a "Base SDK"? for details on how to set it up. You can use my fix-xcode
script to link everything for you every time you upgrade.
The only trick is getting the old SDKs. If you don't have them, you generally need to download old versions of Xcode (still available on developer.apple.com), open the installer package, and hunt around to find the SDK you need.
SDKs can be found within the installer package at:
Xcode.app/Contents/Developer/Platforms/
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