I need to keep down some crabgrass & other weeds in the front lawn, but I don't want to take any chances with the crocus out there (especially since we've had fewer blooms the last couple of years). So I bought a container of Preen to sprinkle around (it's a really small area), and then I'll apply milorganite. Weed B Gone in summer, weed 'n feed in July, and in fall I'll aerate and overseed front and back. I've already got the weed 'n feed and winterizer, so I may as well use them this year... next year I'll switch to milorganite and sprayed weed control.
In the back, I actually encourage clover -- the bunnies and groundhogs seem to prefer it to my "good" garden plants!