I have a gas oven and the lowest temp is about 150 as Julia's oven. I have read where others suggest to use the oven light and that puts off enough heat to dry the cookies, but mine does not have a light. I have never had success with the oven drying method. I got cookies where the icing ran off the cookie! Butter bleed. as someone else mentioned before, was also a problem.
Sometime I find my house is too cold and the cookies do not dry, even over night, in the winter. My house is really old and poorly insulated. I started last year by placing a heater near the cookies, but in front of my fan. The fan blows the hot air over the cookies and this works like a charm! I have found that I cannot have the heater too high or too close as the same problems occur as with the oven method. I have also found that the colors will bleed if the heater is too high or too close. This would not be good for the summer, as it heats up the entire room! I don't get to procrastinate in the summer, as much.