Different folks have different opinions. I believe any leaf, even a partial, torn, half brown one is still better than nothing. There are some interesting data about various growth regulators, some of which promote leaf growth at the expense of fruit growth. As a result, a lot of people avoid allowing too much new leaf growth once the pumpkin is growing. I split the difference. If there is massive die-off near the stump I'll allow a few leaves to fill in, but no new long secondary or tertiary vines. Otherwise, I "leaf" well enough alone.