Q: After breaking my strings and cutting them out, I noticed that my racquet seems to have a hairline crack. Is it OK to still string it?
A: I'm sorry to hear you've got a cracked racquet. I know how attached I am to my frames and it always stinks when you have to retire one of your frames.
I'd say it probably depends on a few factors. First things first, you should understand that there's no way to guarantee the racquet won't break during the stringing process. Stringing creates a significant stress on the frame even under the best circumstances, so stringing a frame that's already compromised is always a risky proposition. Secondly, it's important to understand that a cracked frame—even if it's just a hairline crack—isn't going to play the way it should.
That being said, here's my two cents: since the strings have already been removed, the racquet's currently not good for anything other than decoration anyway. If the crack is as minor as you describe it and you need to have the racquet strung (it's the only racquet you have and you have a match tomorrow, for instance) you might be OK trying to get it strung, as long as the racquet didn't have any sentimental value. If the racquet has any kind of sentimental value at all and you'd be upset if it was broken, then it's probably best to cut your losses and leave it unstrung.
In your question, you said that the crack you noticed was pretty minor. If the crack appeared to be more serious, that would probably change the situation a little bit, for a few reasons. Firstly, if the racquet is seriously cracked, it's definitely not going to play or perform like you'd expect—even with a fresh set of strings. Secondly, the more serious the crack, the greater the chance that the racquet will cave in during stringing or soon after. The bottom line is that it's a bummer to pay for strings and/or labor only to have a racquet that plays poorly or a racquet that ends up totally broken.
But, that's just my personal opinion and how I'd handle it if it was one of my racquets. In the end, it's up to you whether you want to take the risk and roll the dice. If you'll be stringing it yourself, it's totally up to you. If you'll be having someone else string it, then obviously it's between you and your stringer.