The answer is Yes.
The answer is also No.
YES:
At skill level 17, your CM will be -100% cold resists. The minimum resistance that a monster can have is -100%. So if a monster has no cold resistance, anything above level 17 is a waste. Let's assume you go to level 20 and get -115% to resists and a monster has no resists. 0 - 115 = -115 but since lower limit is 100, they are set to -100% cold resistance. In this circumstance, anything above skill level 17 is a waste.
NO:
Some monsters are resistant to cold. Particularly Bosses. So let's take Hell Meph at +75% resistance to cold. If you have skill level 17: +75 - 100 = -25%. This is not nearly maximized. In fact you would need skill level 32 (I believe) to maximize damage against Hell Meph. Skill level 32 gives -175% (I believe) so +75 - 175 = -100%. This has been focusing on PvM only. In PvP I believe there is no limit to -resist, so the more the merrier.
The limit of slvl 17 usually is applied in dual and tri builds where skill points are VERY limited. For example using a Meteorb, every extra point in CM is a point you can't put into a Fire synergy. By assuming +8 skills and only putting 9 points into CM instead of 20, you've saved 11 skill points for Fire Mastery or Fire Bolt.
Taking these pieces of info into account:
PvP: Get as much as you can!
PvM Pure Cold: Max CM to optimize damage against bosses.
PvM Dual/Tri: Stop at slvl 17 (-100%) because you need to spend as many points synergizing other skills as possible.
Hope that clears it up. Also if you read the
Meteorb Guide, it goes into some of this logic along with some computational examples.
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Darzog