Sell your white mods from the 3 common types (square, diamond & circle).
Sell your common-type mods that don't show speed secondaries, unless you have cash to burn. Then upgrade them just enough to see the rest of the secondaries (level 9 for green, 6 for blue, 3 for purple). If they have no speed secondaries at that point, sell them.
Upgrade everything you keep to level 12 to see how the speed plays out. Slice them all to at least blue if they're worth keeping. At that point, only slice mods with +10 or more speed to purple/gold, unless it's a special mod (like a CD set bonus with CD primary triangle) that you can put on a toon with good TM gain, so speed won't matter.
It used to be pretty worthless to slice mods from gold to 6-dot, but that was because it cost 3150 Guild Tokens for the pulse modulators, and whatever you got for 3150 GT was better than whatever the increase was from 5/gold to 6-dot. Now that you're getting pulse modulators for free in different events, you should farm just enough Amplifiers & Capacitors to keep up with your Pulse Modulator intake.
At that point, don't slice your best mods to 6-dot. Instead upgrade the perfect mod for your best character, even if that's not the best mod generally ... after all, you're not going to be able to swap it around (except to other g12s) after the upgrade. Slice those "best fit for my best characters" mods as fast as your pulse mods come in, but remember that when you have a choice, upgrade the mod that only works on that character BEFORE you upgrade the mod that would be amazing on anyone. That way the mods you're most tempted to swap are still able to be swapped for an extra month or two ... and during those couple months, hopefully you're still pulling in event-reward mods and hopefully 1 of those will pan out to be good for every 1 mod you slice to 6-dot and can no longer pass around.
That's how I maintain mod improvement/mod flexibility balance while keeping as many teams as possible at TW/GA-viable speeds (at least +70).