How Sealed Should Valve Seats Be?

SO...bit of a problem...when lapping in the valves I lost focus (talking) when cleaning up the valves from one cylinder after grinding, and have ‘possibly’ put the two pairs down the wrong way round? Just a fleeting moment where I thought ‘s@&t’ I lost concentration there...and then the old ‘did I lock the door’ syndrome kicked in.

I’ve really closely checked and compared the newly lapped rings and feel reasonably confident I’ve got it right, but not 100%?

I’ve tried shining a strong light through the seats, with the valves interchanged - zero light shining through in any configuration, which was annoying.

Finally I felt-tipped the seats and tried a very light spin with some more paste (in the configuration I believed was correct), just to see how things looked, and it looks about right to be fair?

However, I just want to try one more thing...how long should the valves hold back petrol for if they are indeed the right way round?

I guess as the petrol slowly creeps around and down the valves, the Molybdenum grease will get rinsed out of the valve stems...or should it not leak at all?

Anyone know?

 

Many thanks, PS...there appeared to be no differences in the guides. The valves again gave nothing away, they rotated and slid in and out, seating with a satisfying ‘clunk’ in both configurations.
My thought is that the valves could not have been perfect replicas of each other (though they are damnably close), nor their seats so, in the wrong configuration, I’m hoping it might be obvious with a petrol test (even though they appeared to be sealed to light).
Fingers crossed that, if the wrong way round, they’ll leak like a sieve for me!
Glad to hear they should otherwise be petrol tight over a sustained period, though, many thanks for that very much needed insight.