Derailleur hangers are usually made from soft aluminum that will not withstand bending/straightening without cracking. That is so to protect the dropout during this type of mishaps. In my experience, a hanger can be straightened a second time and it will look safe, but once riding, vibration will make any existing crack expand inadvertedly, until the derailleur just falls off along with a chunk of hanger. If it happens while pedalling softly you will feel it and hopefully stop the bike before any further damage occurs, in other situations the derailleur can get caught in the spokes an anything can happen from there.
In my opinion the best option is to get a replacement hanger for your frame, or to get one fabricated.
You may be able to get the replacement from the original frame manufacturer, or from derailleurhanger.com, or from http://wheelsmfg.com/products/derailleur-hangers/all-derailleur-hangers.html. There may be some other options to buy hangers, maybe secondhand...
Derailleur hangers are not too hard to elaborate, most of them are rather flat with perpendicular bolt holes, so they can be made with hacksaw, and a press drill, then a tap and some sandpaper. However, if you are not confident that you can cut the piece, or don't have a source for the aluminum, maybe a local industrial shop may be able to reproduce it. They have all kinds of precision machinery and measurement tools, just bring them the bad hanger, straightened if possible and explain what is it and what you need. In case my English is not good enough and I got the terminology wrong, I'm referring to the kind of shop where they rectify car engine blocks an other engine parts. If they won't accept the job, maybe they can at least provide you a piece of aluminum from an engine block that is damaged beyond repair. I've got replacement parts made like this a couple of times, including bike suspension linkages.
I rushed my answer and did not notice the OP's bike is a CrMo frame with a fixed hanger, so I answered based on a replaceable hanger frame. This first part of the answer is valid for an aluminum replaceable hanger.
As far as I understand, CrMo can be bent a few more times than aluminium, so it should be safe for a second straightening. I've seen the process of "unbending" aluminium hangers, and it is done by fixing the (removable) hangers to a vise and then gently hammering it with a small mallet, attempting to move the bent part only a little at a time. It was explained to me that is is done this way to avoid creating bigger stress in the piece. I guess this is valid for your case too, except maybe that it is more difficult to fix the whole frame to a vise.
It is very important that the bolt hole gets properly aligned, that is, a bolt screwed in the hanger should be perfectly parallel to the wheel/cassette axle, so the derailleur can be properly adjusted. When there is misalignment, some shifts never get to happen smoothly or to stay in mesh.