Yes. That is correct. We need to first install memory on all slots with white release tab the one with black release tabs. this will ensure maximum memory performance. Below link have more details on memory population guidelines   [URL] 
Greetings, In a couple of days, I'm getting 24 new DDR4 RDIMM 2666Mhz 32GB modules in order to upgrade six R640(s) with the current capacity of 256GB. Each server is gonna get four new modules. I'm looking at the manual, and - correct me if I'm wrong - sockets with white release should be populated first (per processor), am I right? At the moment the following sockets are populated: A1, A2, A4, A5 for one processor and B1, B2, B4, B5 for the second one. So following the manual, I should populate A3, A6, and B3, B6 - am I right? Regards,
PowerEdge R640 RAM Upgrade