Your assumption is basically correct across the trade, but not in every case. Crisis management can be more expensive. If it is 95 degrees, the contractors will more than likely be backed up with service calls and immediate installs. The likelihood of being able to negotiate an install price under these conditions is slim.
Also, beyond the economics of your project is the fact that AC install is better for you when you and your family are not personally inconvenienced by being without cooling for a day or two.
Winter is not out of the question, but the biggest issue is that to properly start up a new AC system, you really need a warm day (65+), and this usually requires a return trip for the installing contractor in the spring for final start up and that can add to their expense.
Good luck to you with your project.