I've put mulch down at all times of year, but for maximum visual value of fresh laid mulch as well as better weed management, I think spring is best so you catch them before they come up. Be aware that weed killer is poisonous, so I would not advise that you "drench" your soil with it. Also, be aware that if you have a broad expanse of unplanted land, weeds will grow there even with fresh mulch (nature abhors a vacuum). You are better off planting something that is low maintenance and will help choke out any weeds that try to grow, which won't be "no" work but will be "less" work. Things like grasses and ground covers (vinca, pachysandra, ivy) would be good choices.

All landscaping companies around here always mulch in May.

Whenever mine are looking raggy, I rake them flat (into the yard, mow over it, it's gone within a week), layer newspaper thickly, then put new mulch over it. Any time of year.

The newspaper bio-degrades nicely, keeps weeds out, and doesn't have the awful wrinkling/movement that can be had with fabric..as well as being eco-friendly.