about power cords (not chords)?

At the sort of power draw of those units, don't worry about it.

The danger actually comes when they're coiled (not folded) and under high power draw - the coil acts as an inductor and can generate a lot of heat.

I once saw a band's wind-up extension cord literally melt when they left about 3/4 of it wound up in the casing and ran the whole backline and PA from it (probably around 10 amps @240V).

Preamps and effects units draw in the milliamp range, no need to worry at all.

In fact, purposely coiling the cables up very neatly may help reduce line noise.

 

Just don't wrap them TOGETHER with the signal \tuff... if you want to keep noise to a minimum.
gregc

 

Also, a coil acts as an antenna, in this case broadcasting the 60Hz AC (although the twisted conductors in most power cables help minimize this).

For the same reason, neatly coiling the signal cords should be avoided.