Leaving the coolant system dry

Just started work on a major service on the 636 (clearances, carb balance, etc) and I've given up for the day. Carbs are off and covered but I've drained the coolant. Now I know its dangerous to leave radiators empty but does anyone have an idea just how dangerous it is. Should I consider keeping it full for the 2 or 3 days I'll be working on it?