If you kill the weeds now, and then wait, I would think you'll have some new weeds spring up by fall. I did that on a strip that I intended on rocking with landscape. I used round up, killed the grass, but then by the end of the summer I had a whole new kind of weed growing. I might try and do it all at once. FALL being the best time to seed because of the cooler temps. Spring is fine to seed if you want to spend a little more time with the care of it all.