Photoshop allows you to save in numerous format, and the original .mrw should not be disturbed in anyway (that your original negative copy). You can not save back in .mrw since this is not a valid file format, and is not a raw file from the camera anymore!
Tiff is the most universal, but keep the file as native Photoshop file is your best best since it preserve the layers information. But why? All theses format are very large beside your .mrw. In my opinion create your editings steps as an action and save it (let's call it MRW2Tiff for example). Everytime you need a copy or for converting other .mrw files (they should be very close unless the exposure is bad) just run this action again (the action file is very small)!
Remember you can duplicate this digital copy again anytime...