Theoretically, nothing is "safe". There's a chance to fail going to +1 and I have heard of people breaking items going to +2. Whether this is true or not, I have no idea. I do know that it has never happened to me and I've played for a long, long time. Far too long, maybe. I consider 1-3 "safe". You may fail, but the breakage (destroyed) rate is so low it is negligible. Anything beyond that can and will possibly fail and be destroyed at the worst possible times. Here's a few guidelines. If you're going for str/damage equipment, don't bother upgrading your hat (it just gives int/vit and defense). Gloves are a must, leave your boots and pants alone, and your top should maybe be considered for +7 or higher. Now, if any of the ones you *should* upgrade are extremely valuable, consider leaving them at +0 entirely to avoid the rare destruction-at-plus-2 scenario.
"If someone asks you whether or not you're ticklish, it doesn't matter what you say, they're gonna touch you. If you don't wanna be touched, you gotta say something like, 'I have diarrhea. And yes, I'm very ticklish.'"