@kistrand To piggy back on this, plan for the worst case scenario, but don't always act like the worst case scenario is the case. For example, if your opponent is on blue, has drawn some and has 5 cards in hand - he MAY have FoW, so you have to carefully consider casting that clutch spell. But if you DON'T cast that spell, and have nothing you could topdeck to push the spell through next turn, then not only did you virtually give them FoW in their hand, but you also gave them time to dig more and actually find FoW.
Sometimes they have the stop, sometimes they don't. Sometimes you cast your spell and they counter and it seems crippling (but if you held it in your hand, they'd STILL have the stop, so does it matter?). If you never cast the spell out of fear of a counter, then they ALWAYS have the stop, whether a counter is in their hand or not.