Pet peeve: when people take phrases intended for one thing and uses them for a slightly modified purpose. I take Branson's "screw it, let's do it" mantra as something entreprenurial, a call to break the rules (and stuff like that). Using it in this purpose feels more like "screw it, let's work even though I'd rather stay in bed", which is different emotionally, and kind of saturates the sayings power.
(Maybe nikes "Just do it" would fit better, but I get that its associations to big corporations isn't as well suited for the startup atmosphere we're surrounded by.)
Especially as the root of that phrase is in really wanting to do something, in the face of overwhelming external pressure to not do it (I think he coined it when launching Virgin Atlantic and noone thought the business plan looked solid).
(Maybe nikes "Just do it" would fit better, but I get that its associations to big corporations isn't as well suited for the startup atmosphere we're surrounded by.)