This opens up the old debate about whether if you are in open FIR you should take a service. There are those that say no, there are those that say yes, and there are those that say only if a radar service is on offer.
Leaving aside the debate if you are in VMC, I am with the first camp if you are in IMC. This means I am going to take a service from who ever can give me one, but obviously if I can get a radar service I will take that over a basic service.
I could explain why but that is another debate.
So given that you can nearly always get at least a basic service and given that if you are in IMC you are IFR then I dont think it is acceptable to switch to IFR without telling the obvious service provider. Clearly it is also not acceptable having told the service provider where you are and at what level not to report any change in level or course.
The fact of the matter is that I find when conditions are IMC the number of aircraft about in open FIR is relatively few - the world and his wife are in the bar. Personally I would quite like to have an idea where you are even if you believe the big sky will keep us apart (although thinking about it I dont give a damn as long as you are transponding
). You might think the same if you had nearly met two aircraft in and out of IMC that were talking to no one.
Clearly to fly a procedural approach you are considered to be IFR so I am not sure that strictly you need to declare you are. The controller will always assume this to be the case. There are circumstances where you might want to fly an approach entirely in VMC (for example in France without an IR). It remains my understanding that the controller is entitled to accept you at his discretion in the procedure on a VMC declaration - in other words a contract between you and the controller that you will maintain VMC at all times.