What is the meaning of the epigraph in relation to the themes of the story, such as tempting fate?

The epigraph of "The Monkey's Paw" is "Be careful what you wish for, you may receive it." This old saying neatly sums up the whole story in a nutshell, because the main action concerns the Whites' making three wishes on the monkey's paw, each of which results in unforeseen consequences. The Whites do this despite the fact that Sergeant-Major Morris explicitly warns them against it and even throws the paw onto the fire to prevent them from messing around with its dark and deadly powers.
But the paw is retrieved from the fire, and the Whites proceed to make their three wishes, treating the whole thing as a bit of a harmless joke. In doing so, however, they're making a big mistake: they are tempting fate. After each of their three wishes, they do indeed get what they asked for, but not in the way that they'd hoped. This is what the epigraph means: the Whites, in making their wishes, were not careful in thinking through the potentially tragic consequences that would accompany those wishes being fulfilled.