We gotta start most of these specific talks sooner than not.
With the Final Update underway, many more kicker cards from Dominaria are coming into sights.
From my own evaluation, I feel like the magic number is +2 cost at most. +3 really slows a card down, and most of the +4, even on 2cmc bodies make them underwheling after their effect has taken effect.
2cmc 1/3, but a decent ability for +4 more makes them Borderling at 6cmc, but a 1/3 Vanilla body doesn't advance the board much.
It doesn't matter if the kicker cost is +2 or +5, what matters is wheather you're getting a good rate. If the base made or the kicked mode is aggressively costed, and the other mode is at least fairly costed, then the flexibility can push it over the line.
Into the Roil vs Repulse is a decent thought exercise as neither option is strictly better, but both are perfectly cubeable in peasant (although I'm not currently running either).
i think even maro said most players usually go for value
Maro did say this, but he also said that was wrong. In the example he gave, he's referring to Kavu Titan and specifically says that it was often wrong to not just run it out as a bear. And he talks about how players evaluate cards incorrectly based on extra text without looking at how valuable the base mode actually is. To n00b1n8R's point, curving out wins games. If you have something better to do each turn before you get the mana to cast the card for full value, then it's probably correct to wait. If not and casting the card in its base mode will affect the board somehow, then it's normally going to be correct to just run it out and keep the game flowing.
This story isn't really about design but it's become such a classic story (and it involves kicker) that I felt obliged to tell it once again. (And yes, I know I told it during my Invasion Week article, Body Snatchers of the Invasion.) During Invasion development I played frequently in the FFL (the Future Future League, where we play many months ahead to try and figure out what the future environment will be like). Because I wasn't the best of deck builders (at least a Spike deck builder; I could hold my own in a Johnny world), I usually got decks from other developers. Randy Buehler was running the FFL at the time, and he gave me a red-green deck to play. In it were four Grizzly Bears. I didn't think anything of it at the time. I then went on to go 4-0 that week. I was the only person with an undefeated record.
During my last match, Randy watched. At some point late in one of the games, I played my Grizzly Bears. After the match was over, Randy informed me that the Grizzly Bears weren't actually Grizzly Bears. They were proxies for Kavu Titan. Armed with this information, the next week I went 2-2. Suffice to say that the R&D guys thought this was the funniest thing ever. The lesson I learned was the importance of using the Kavu Titans as 2/2s. When I was aggressive with Kavu Titan, my deck performed much better. When I waited to maximize the Kavu Titans as 5/5 creatures, I lost much of the aggression of my deck and thus some of its power. Week 3, I went 4-0 again.