In a healthy lawn, weeds should be minimized. It's only in an unhealthy lawn where weeds become major problems.
If you have a thick, green lawn, then weeds should not be a problem, but they will show up from year to year. Instead of putting down a selective herbicide over the entire lawn, it is better to spot treat the occasional weed with a small hand sprayer.
Not only is this less expensive, but it is better for the entire yard. Herbicides, even selective herbicides, put stress on turf grass plants. Avoiding the stress on the entire lawn is better.
Spot treat whenever possible. Early spring or fall treatments are usually best. Younger weeds are easier to control than mature weeds.
Precautions when applying herbicides:
Do not apply during hot days because they may injure the lawn.
Do not apply during the rain. Broad-leaf herbicides require 24 hours of dry weather to ensure good uptake by the plant.
Delay mowing or irrigation for several days after application.
Do not spray under windy conditions.
Do not use 2,4-D or dicamba products on newly seeded lawns.