Installing Xcode betas on your computer is probably fine. It might mess with other projects on your computer, though, but those are backed up by version control, right? So you're fine - recovery is easy in the unlikely event that things go wrong.
Installing the iOS beta SDK on your computer is also probably fine, for the same reason. I'd be more shocked if this broke things than if Xcode beta did.
Installing an iOS beta on your personal iOS device is a terrible idea. I'd suggest you get another, dedicated test device, if you must have the latest beta running on a physical device to test your applications. If you really must put it on your personal device, make sure to at least back everything up before you do so. Be prepared for all of your apps, everything in every app, your pictures, music, videos, chat logs, emails... everything... be prepared for all of it to vanish.
Installing an OS X beta on your personal Mac is an arguably worse idea than installing an iOS beta on your personal iOS device, because odds are, if the beta bricks your Mac you'll be losing a lot more than if a beta bricks your iOS device - you'll probably lose a lot of files forever because few people actually have remote backups of their entire computer. Getting a replacement Mac will probably take longer than getting a replacement iOS device, which means you'll set back your development process by quite a bit. Plus a replacement Mac will cost a lot more than a replacement iOS device in most cases, unless we're talking about the cheapest Mac Mini vs the most expensive iPad.