How long before WWDC are old models usually put on sale?
I am not aware of public price cuts prior to WWDC. At least not with today's Apple.

The practice you mention may no longer be in place, it may be limited to some stores, or it could be country/region specific.

Apple's computer products tend to maintain the same price for their lifetime. iOS devices can attract discounts over time – their more predictable update cycle helps with this.