Rear sockets U182 and U183 are for 27C220 type chips and the front sockets U180 and U181 are for 27C400 type chips. You can only have one set populated at a time.
Early A3000s need a ROM tower if U180 and U181 are used. It is unclear to me which revisions truly need it. 7.1 definitely needs it and 9.0 definitely does not, but this is not all of the revisions. :-)
U182 and U183 are always the correct 27C220 pinout in A3000 motherboards.
1.4 ROMs do not stop you from using a Z3 memory card, I have this combination in my A3000, but I forget which revision the board is now. It has a ROM tower and IIRC it is 7.x.
I found a few more data points here:
https://www.amigalove.com/viewtopic.php?t=13397.3 boards shipped with ROM towers, 8.9 did not need a ROM tower.
Most likely 6.9 will need a ROM tower too if 27C400 style ROMs are to be used.
An educated guess is that all of the people who burn KS ROMs for sale currently will do 27C400 chips and aren't aware of the tower-less 27C220 possibility for older A3000 revisions.
