This is something that really should be clearly stated in the install instructions, and it may differ per phone model;
I guess that at least for Fairphone 3 and on, stock ROM upgrade is not required, as long as /e/ have the relevant version of firmware;
“/e/ gets the relevant firmware parts from Fairphone and includes them.”
from Failed update, why (q0.17, FP3) - #4 by AnotherElk
Edit: speaking only for the FP3 - at the moment, for both the Q and R images, you can skip stock and go directly. As fairphone.com didn’t publish an official A11 image - they, /e/ 0.18 itself (since #3670) and lineage (17+18) all flash the same firmware partitions to A.0107 currently.
Only difference is - if you use the FP3_flashall script, it writes to A and B slot, while the OTA way, you flash (in principle) the inactive slot (should confirm this some time 
). But this is easy to overcome, you can flash to parity (dd src dst) between a/b slots in fastboot at any time (given an unlocked bootloader).
Well, we’ll see when Fairphone OS Android 11 comes – it was vaguely scheduled for “second half of 2021”. As @molej said, /e/ should include Fairphone’s firmware parts in their upgrade, but then again, I will have to do a clean install for /e/OS Android 11 anyway, so turning to Fairphone OS Android 11 before /e/OS Android 11 won’t be so much extra effort (if the time window in between the release dates of both isn’t too big).
The entire information provision regarding /e/ is rather gnarly and cluttered.
Take for example the releases of R , which roms are already released , I can’t figure it out .
It seems like /e/ is in all ranks , like walking in a maze.
No /R/ ROMs have been released by /e/ yet, except test builds, targeted at the groups who maintain and/or test specific device ROMs. @Manoj has included a link to the release plan in the first post
Still no OTA update 0.17 → 0.18 nougat for my One Plus One visible. Is that correct for now or should i do “things”? 
Seems correct … https://images.ecloud.global/dev/bacon/