To clarify- you do NOT need a flight plan to be provided with a flight information service and alerting service in class g. This is provided to all aircraft who have filed a flight plan, are known to be in distress or otherwise known to atc...so as soon as you call with your call sign you are known traffic and will be provided with a FIS and alerting service.
BUT just remember to advise ATC when you are finished with the service either before you land or afterwards on the ground by landline so atc/guards dont have to go searching for you 30 mins after last transmission or postion report. This applies if you just pop up for a local flight and you just call to check qnh or transponder or for traffic too.
A flight plan is required, amongst othertimes , when you want to enter class C even if you just want to transit thru and not land. If you want to land at a class C airport then a plan filed will smooth your way and dont forget you need another plan to get back out!
The other thing is that atc service provided in the fir/class G does not mean you need to file a flight plan because its not a control service its only an information service and as such atc will NOT seperate you from terrain or traffic. They dont even provide a radar service they just use the radar to help plot where the aircraft are.
And finally from experience both ifr and vfr can operate in class g and there is no special attention given to the ifr traffic. They might be ifr but its still see and avoid!
I always tune in 127.5 even if its just to listen. the service is great with the heads up about traffic --sometimes its miles away sometimes its damn close but if i had a radio i would use it even tho the plane flies fine without it!