Not necessarily. The best time to treat for grubs in the late summer or early fall when you are likely to remove the young grubs and stop repopulation. You also have a short window of time in the early springtime, before the grubs get too big and begin to pupate. It can be tricky in this second window to achieve an effective treatment for grubs.
If you have missed these windows, though, it does not mean you need to surrender your lawn to the grubs! There are plenty of products that work right through until September, eradicating your grubs.
It is worth noting that preventative treatments will go a long way in reducing and preventing grubs from entering your lawn. These treatments will work all year round. If you have not used these, treat your grubs in the late summer to stop the next generation of grubs from wreaking havoc on your lawn!