Just came across this thread today.

It's odd, but another thread (curse my absentmindedness) advises giving carrots, petting and getting them familiar with you. With this in mind, since I have picked up 1.2.0 from Jon, I have been feeding them slices of mango by hand..and petting them, so far no bites and they do let me pet them. (I keep them in the kitchen for now..until they start breeding and/or are used to me)

I started off my rats the same way (the new ones that come in are shy), feeding them treats by hand..using GLOVES to handle if I was uncomfortable with a rat until I got used to them. I can handle my rats without gloves, have only been nipped 2 times (if I recall correctly) by new rats and I promptly fed the nipper off. The rats I have now, can be fed by hand and patted briefly *I have never tried longer and I have no desire to hold them*

I can't remember where I got that care sheet, sorry.