I think with a sample size that small it's a little hasty to call it at 5.
I think the main takeaway is to avoid dying while shutting down the enemy rather than "keep it under 5 deaths".
Recent example: I went 10/4 in a Veigar game today, and we were performing pretty well and winning hard. Then my team separated (adc was separate from team, got pwned by Jax, I was zoned out as well and couldn't enter the fight at all) and got the shit kicked out of them in a 3v4 and I couldn't really defend the base against 5 champions (Jax, Riven, Lucian, Taric, Kha Zix) so I had to let them take the inhib tower, inhib, nexus turrets, and nexus.
Long story short; people are really bad at late decision making. If you want to leave lower MMR, you MUST WIN EARLY. The longer the game goes, the more likely you are to lose.
EDIT: Another example! Winning hard, Ziggs gets caught, we lose the 4v5, end of story.