Always hired them, since you get only one shot at doing so, whilst getting extra 500 gold is very simply and there is more than one way, at any time to re-gain it. I use Helena quite a lot during the mission 4-5 stretch, whilst Flint I use on occaison to liven things up a litte- and it is rather ironic that he is, ultimately, the fastest character you have on your team (max speed 13, I believe, with steam engine and capacitor). In any way, it's worth keeping them as alternatives, just in case the player gets bored or decides to change strategy.