It's pretty rare for me to either scratch the ceiling, or NOT have a blade in hand when I'm home. The wife is used to it, at this point. She tells me it's strangely comforting to have me either practicing a cut, or sharpening a blade pretty much all the time. My friends always know I'm feeling better after I've been sick when I've got a blade of some sort in hand and am playing with it again.
I think the main thing is that you really need to be very aware of where the blade is when you're practicing, especially if you're using something low quality which might break. You'd feel like a real tool if you misjudged a swing, or had someone come up behind you, or the blade broke when you hit the ceiling, and you injured or killed someone.
The other issue, of course, is that even with short ceilings, it's to your advantage to keep your blade close to your body. The wider your swings, the easier it is for an opponent to penetrate your guard. You want your cuts to be economical and to return quickly to a guard stance so you can block or parry as necessary. And you want to know exactly how far you can reach on a hit.
It's probably safest to practice outside, but really, the best way to learn, in my experience, is to start slow, and really watch the blade so that you gradually gain a feel for it. Plus, you want to know how it feels to move with perfect form, which is in some ways easier slow. Build your speed up gradually, until the sword starts to feel like an extension of your body. Pull the power for your hits from your hips, legs, and torso, not the arms, and you'll find you can swing a sword much more easily in an enclosed space without damaging your ceilings, etc, and you'll get more power to boot than you would with your arms.
What kind of sword are you using these days? Techniques vary widely depending on the kind of blade you're using. A really long blade, like a longsword or a dai-katana, or worse, a claymore, should probably not be used indoors ever. But short swords, one hand swords, etc, can be used fairly easily in an enclosed space, even fighting in a hallway.
