Pre heating your engine is never a bad idea. Your engine will last the longest that way. The biggest wear an engine endures is during startup. Its never good jamming the piston into the taper of the sleeve when the sleeve is cold. The taper is what gives the motor its compression characteristics. The sleeve will expand under heat and pressure and will expand faster and farther than the aluminum piston and the bore will naturally become square with the piston when it comes to temp. By preheating the block you are expanding the sleeve enough so the piston doesn't have to wedge itself in there at TDC during start up minimizing wear thus making your engine last longer. I had an S5 two years ago that went 2 gallons and still had pinch similar to when it was brand new. those RB motors are a great quality engine for the money and they are even better with a nice port on them. I agree with the 180 degree mark. Thats definately a good place to start. I just heat it up for about 30-45 seconds until the head is hot to the touch and then spin it. Just make sure the piston is at BDC during start up and cool down procedures as not to damage the piston crown. Good Luck!!
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