Will a 5V crystal oscillator oscillate with a 3.3V supply?
Maybe. As in, it's not guaranteed to, but it's also not guaranteed not to. 

And it might even do something really devious, like work when you are looking at it and not when you aren't. 

Or you could get a bunch which work quite well and use them in prototypes, and then your contract manufacturer could get them from a different factory lot and they'd all fail to run in this out of spec condition. 

In other words don't do that unless you plan to take over the manufacturing test/QA role usually performed by the part manufacturer.  For extreme applications where there are simply no parts with official specifications which meet the need, this is sometimes done, but it's hopefully done with full awareness of the risks and shift in responsibility.