IMO, if you had to pick a specific time use them, it would be pre, intra and post. BCAAs are going to spike proteins synthesis quickly and facilitate recovery, as will glutamine. I also include a complete protein as well i those drinks (pepto-pro).
You can also have BCAAs between meals, which I found to be helpful when dieting to preserve muscle and prevent any kinds of cravings.
I keep a BCAA drink on my nightstand and drink it first thing in the morning. I figure why not get protein synthesis kicked off ASAP and stop any catabolism.