I always convert them unless it's something special*, Donald.
The trick with them is to make sure the nylon head and cone are a close match to the cylinder end and the O ring is a nice sliding fit in the cylinder.
I check that the nylon head and cone are a good match to the cylinder end by putting Tipex on the front of the piston and rotating it against the end of the cylinder --- if the Tipex is wiped off all faces it's good to go.
With an O ring piston head fitted you can normally get away with less Spring for the same power output as you get from a leather seal --- the conversion is fully reversible as well so you're not irreversibly altering the gun.
All the best Mick
* - I serviced an Airsporter as a favour for an old boy and it turned out that it was a transitional MK1/MK2 model so I left that one as it came from the factory.