I realise I'm somewhat late on this discussion, and also somewhat unknown around these parts, but I find myself pressing down on the back of my knives blades with my thumb almost every time I use them, and especially when whittling. I find choking up in that regard to be very very useful. Although, naturally, with slipjoints one doesn't use the thumb to apply pressure per se (doing so would probably result in a rather nasty nick in your index finger I imagine...not speaking from experience at all...) but rather it provides a quit convenient brace from which to lever when doing detailed work. More of a support than anything else really.
Anyway, take it for what it's worth, cheers,
James.