As far as Walkers are concerned, I try to play with the ones that seem most interesting and/or most interactive. My cube is pretty high powered but avoids most GRBS, so stuff like
Liliana of the Veil and
Jace, the Mind Sculptor aren't majorly game-warping. I also like playing with powerful cards so I'm probably never going to cut either of them. The key is to keep them in check with the limits of your environment. If the match is dead even with interaction on both sides, does a walker just completely swing it in one person's favor after a single activation? Can the walker be dealt with effectively via efficient threats? Do you have enough evasive creatures/ways to attack the walker with damage (or outright destroy effects)? These are reasons why I'll never run
Karn Liberated or
Ugin, the Spirit Dragon.
My aggro suite has the typical RDW available if someone wants to force it, but stuff like
Champion of the Parish Wx builds or Recursive Black Aggro with
Gravecrawlers and
Bloodsoaked Champions aided by double Bonesplitter really go a long way towards applying enough pressure to keep Walkers in check. I'm also a fan of trying out pretty much every new card that comes out with "Destroy Target Planeswalker" text. White has the small threats to apply pressure, Blue has the counterspells + bounce effects, Black has the kill effects, Red has the Aggro + burn for reach, and Green has the ability to power out early threats to apply pressure. As long as you can tailor your environment to reasonably deal with the walkers you choose to run, there shouldn't be too much of an issue with most walkers.
I've definitely toned it back on total walkers with recent iterations of my cube, I much prefer those that shine in certain strategies rather than one that is universally great to just splash for (like
Ajani, Caller of the Pride for aggro. That was the problem with
Gideon Jura. As much as I love the card and think it's completely badass, that shit swings the game so quickly to one side. It's frustrating as hell to go up against. I'm currently at 14/420 for Walkers, they aren't in every pool we play with either, so I'm fine with that.