Ian – how does your coffee machine detect when it has used all the water in the tank for a given "dose" of coffee? If it shuts off due to temp rise in the element when the water is used up and boils away, as many coffee makers do, a continuous feed float valve may not work. The water would continue to replenish from the float valve, and a) the coffee would get weaker – BIG problem b) the carafe would overflow – messy but less of a problem. 
It might be safer and easier just to plumb a supply tube above the tank with a momentary action pushbutton valve that opens only while your finger is on it. That way the tank could be easily filled and flow when full could be stopped fast. No one could leave the tap on causing an overflow either. For hygiene, be sure and leave an air gap of an inch or so from top of water level in the tank to the fill outlet tube end. The air gap prevents any backflow of "dirty" water into the house / town water supply.
A safety overflow pipe from tank to sink would be a nice to have feature against overfilling, but again this must be planned carefully for an air gap above any possible standing water in a sink. The overflow must not be plumbed direct to a drain or sewer gas or backflow could contaminate the tank and water in it.