Some of the earliest electric basses had a thumb rest above the pickup. Most basses don’t have this so the best place to position your thumb is on the pickup itself.

Use this point as a pivot to move closer to the neck for a softer tone, further back towards the bridge for more attack, or stay close to the pickup for general playing. When playing fast runs it is often better to play closer to the bridge so there is less movement in the strings. When playing on the D and G strings you can also try resting your thumb on the E or A strings.