The centreline should be primed even for 'dry' prods, so that the hose 'flies right' rather than 'straightening up'. Once the receiver makes contact, even with the 'tanker end' closed off, the hose will usually empty even if just into the receiver's probe and AAR inlet gallery. Unless, that is, there's a method of closing off the receiver's probe completely.
Hence my reference to 'damp' (or 'moist'...
) contacts!
There's also a slightly esoteric reason for such contacts not to be completely 'dry' - involving the HDU fuel valve and the normal light sequence for the receiver. But there's no universal STANAG (yet) for this.
Not necessary for wing hoses though - and usually not even possible (except for a pointless modification on one tanker type...
) ). If receivers miss a wing hose, they'll often call for the wing hoses to be 'primed' as they think that the tanker is at fault..... Such calls are usually met with "Roger" and nothing is done - but it keeps the frustrated FJ prodder happy!