But i tend not to read the emails i get from merchants on a friday afternoon as much as earlier in the week.
Also as is sometimes the case what's in the email doesn't often manifest itself on the merchant/network - so at least an email within the working week rather than the end of it means any problems can be addressed sooner rather than after the weekend.
I guess there isn't a "best time" - but there must be a better time than friday afternoon!
Worth pointing out that some networks screen the e-mails and merchants have no real control over when they are sent out. Maybe the networks would place a 'send on' date field??