Fuel or Time?

On my way back to base the other day, i had quite a bit of fuel left. Ive always sailed at around 7knots for fuel economy, but i thought whats the point in going slow when im heading home anyway? i was out of torps and ammo.

This brings to mind the questions: What was actual uboat commanders recomended to do?

A. Head home fast, not so fuel conservative, as this meant less time at sea without fighting capability

B. Head home reasonably slow, meant saving fuel for further patrols