 If you spray a big weed that doesn't die, it now has a chance to become resistant. However, if you don't spray a big weed, you know your yield will get hurt and that weed will go to seed, leading to more issues in the future, which is why almost everyone sprays even when weeds are beyond labeled height. Our advice if you're going to spray big weeds is to increase the use rate of your herbicide to the maximum labeled rate. If you can, up your adjuvant package to include crop oil or methylated seed oil and add a tank mix partner.