im currently running aftermarket alloys, with Falken 195/50/15's i asked my insurance company about extra costs as the standard wheels were 14" steels and they said there would be no change so i told them to add the alloys to the insurance

(total cost was £400 for the alloys + tyres fitted..alloys cost without tyres would have been £219).
Really i would go for it, there might be a slight offset in the sensors but at the end of the day its worth going for because of the savings (ok so my tyres in future will only be £5 each cheaper..but hey £20 saving on 4 tyres is £20)