I clip them right after stringing and tuning. High Tension strings. I never had a problem with slipping strings. Mine do not become shorter, but longer.
Maybe it's because I'm an evil heretic: I never ever started stringing a classical guitar at the bridge. I always start at the roller and have never tried doing it the other way.
Decades of experience with electric bass and steel string guitar taught me two things: Never cross strings on the roller and wind up as little as possible to achieve the best tuning stability.
Some of you might have seen this picture before, but nevertheless, once again: This is how I fix strings that don't slip. I cut the ends at the bridge, as soon as they're strung up.
Very slippery thin Carbons might move a few millimetres before they're fully tuned up, so the sharp bended part can be twice the diameter of the capstan (cut the overcame after being in tune). Bass strings do not need more than one winding on the roller. Some bass strings have a soft end. I always use that end for the roller. And some 6th strings are too thick to put them twice into the hole: I make a knot.
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