Wipe out eMMC before flashing new OS?

[vajak]

In fact, I didn't do it, and after booting from the eMMC lsblk reports, in addition to the expected mmcblk2p1 and mmcblk2p2, two additional entries: mmcblk2boot0 and mmcblk2boot1. I wonder if those are left over from the previous OS...

According to wiki.pine64.org/wiki/Pinebook_Pro#eMMC_information:

"The eMMC storage will show up as multiple block devices:

• mmcblk1boot0 - eMMC standard boot0 partition, may be 4MB
  
• mmcblk1boot1 - eMMC standard boot1 partition, may be 4MB
  
• mmcblk1rpmb - eMMC standard secure data partition, may be 16MB
  
• mmcblk1 - This block contains the user areas
  

Only the last is usable as regular storage device in the Pinebook Pro. [...]"

The Wiki contains an overwhelming wealth of information and is a bit hard to digest on the fly. 
Takes some time and patience to get acquainted with the information it offers.