It’s in the National order. Just depends on if the controller thinks you are having weather difficulties or not. 10-2-8 a. If a VFR aircraft requests radar assistance when it encounters or is about to encounter IFR weather conditions, ask the pilot if he/she is qualified for and capable of conducting IFR flight. b. If the pilot states he/she is qualified for and capable of IFR flight, request him/her to file an IFR flight plan and then issue clearance to destination airport, as appropriate. I do this semi-regularly and have never been sent off to file. When I’m controlling I don’t send them off either and just input it at the scope. Some (seems like center folks) are told that you need to send them off to have all the emergency info on file (i.e. fuel, SOB, etc).