Will winding a single DC cable on a spool cause any issues?
Loops with a bit over 1 turn are similar to straight cable as far as current handling is concerned. 18AWG 2 core cable is rated for ~7 A, well above your maximum current, so heating should not be a problem.
Resistance is ~21 mΩ per meter, so the extra voltage drop will be ~ 0.5 A * 0.021 Ω * 0.3 m * 2 conductors = ~6 mV, which is insignificant compared to 5 V.
Inductance of the loops is about 0.2 μH, which will probably be insignificant in a cable carrying DC power.
At 500mA there's no worry about cable heating. A common situation where unspooling cable matters is for a vacuum cleaner for example, where the cable contains as little copper as possible (because copper costs money) and is tightly wound on a spool inside the cleaner without any air cooling. In your case, no worries.
A loop of wire creates inductance, but a loop of cable which contains both hot and ground does not add inductance in series with your power supply, because the current in each wire is opposite, so that cancels out the magnetic flux. Low inductance wound resistors are made this way too. So you shouldn't worry about inductance either.
The winding will create common mode inductance, a bit like a ferrite clamp on the cable, which isn't a problem.
The only issue would be if the coil is wound around something that emits a magnetic field, like a transformer, or if it is wound along another wire going to a high current device like a motor driven by PWM. So you shouldn't bundle all the cables together and then coil them all in the same loop, that would create an air core transformer. Again, only a problem if cables or wires generating a strong magnetic field from AC current is bundled with sensitive analog signals. For 5V power supply, shouldn't matter.
The answer could be divided into these parts:
Current and power lost. A cable have a resistance and the power lost will cause heat and a winded cable have no means to dissipate the heat from the inner loops. This is unlikely with AWG18, 5v and 500mA.
High frequencies. A loop is a inductance and can cause undesired effects. Without knowing what kind of circuit...
Magnetic A electromagnets parts is a coil and a magnetic core. Is there any magnetic sensors that could be affected?
So with other words it depends.