Bottom line, coiling the cord should not cause any problems, unless you have some pretty extreme conditions.
Heating of the cord could be a problem, if the ambient temperature around the cord is significantly high, and the current draw through the cord is also high. But, under normal circumstances it should not be a problem. As mentioned in other posts, from a theoretical perspective, the inductive reactance of a coiled cord versus one that is not coiled would cause some losses. But, at 60hz (normal power line frequency), these losses would require very sensitive equipment just to measure them. Actually, the inductive reactance would more than likely be offset by the capacitive reactance of the conductors in the cord that essentially run parallel to each other.
