It seemed like the scoring worked such that the best bet was to get an even mix of positive and negative cards for as long as possible, and then standing when it came chance that you could bust. The stretchy score and telepathy upgrades are a powerful combo.
Some of the upgrades seemed to be more of hindrances, like the ones that made face cards all positive or all negative, since that would unbalance the card ratio and shorten strings of cards.
I ended up going until I crashed the game trying to skip an upgrade screen with stuff I didn't want, having regularly been getting 300-500 point hands. (I guess I should have taken the face-card removal upgrade, that'd reduce the average point swing of the cards and thus increase the length of card stacks)
And, for some reason, half the score displayed off the screen for me playing in browser.