It doesn't really matter when your option is exercised. You bought a stock for price K - what the stock it worth at the time of exercise doesn't matter.
Suppose you have a call option with a strike of 100. Right now the stock is at 110 so you decide to exercise the option, which happens immediately. 10 minutes later, the stock drops to 90. You paid 100 for a stock that's worth 90 for a loss of 10.
Now instead of exercising immediately, suppose the option doesn't get exercised for 10 minutes. You still paid 100 for a stock that's worth 90 for a loss of 10. The fact that it was worth 100 is irrelevant. You're in the same place financially either way.
As others have mentioned, selling the option to close is almost always a better decision (since options have time value above the difference between the strike and underlying price).