OK, I'm an electrical engineer and have been designing radars and electronics for 40 years.
Given a properly operating and designed radio and a properly designed and installed antenna, the length of the coax does not matter, only it's impedance (aka Z). Z does not vary with length. Given the previous asumptions, coiling the coax has no effect. The problem comes when the radio is not a true 50 ohm source or the antenna is not set up to be a 50 ohm load. If both ends are a good 50 ohms and the coax is correct, 100% of the power should be radiated out the antenna. This is equivelent to 1:1 SWR and is ideal. A high SWR means that most likely the antenna is not operating at 50 ohms and needs to be adjusted. The Z of the antenna changes based upon it surroundings, whether it is mounted on a flat surface, a curved surface, a rack, or if it has other metal near. That is why antenna's are adjustable. Don't worry about the length, just adjust the SWR to the lowest possible number. If the numbers are crap, start looking at how the transmitter is grounded and how the antenna is mounted and grounded.
In theory, the shortest coax yields the lowest loss, but at 27mhz, it is a small difference. The reason why the 18' number comes into play has to do with wavelength theory and it can help if the load is badly mismatched, ie not 50 ohms.