Does this mean that on machines with 1 MiB of RAM some parts of it (and hence the associated DRAM ICs) will never be used?
No. For one, at this time Machines did not come with 1 MiB, but 640 KiB at max. Even when 1 MiBit chips where used, they where usually of the 256x4 kind, so 4 of them made up 512 KiB plus another 4 64x4 chips (256 KiBit) adding the remaining 128 KiB. For 8086/88/186 machines there was no reason in having more than 640KiB (*1).
With upcoming 80286 machines usage for more than 640 KiB became possible. Early AT still had only 640 KiB Base Memory and everything above 1 MiB was to be added with add on cards. It wasn't until Chips and Technologies came up with their NEAT chipset. NEAT being an Acronym for 'New Enhanced AT'. Beside continuing the further integration of prior discrete components like Bus Controllers, Timers or DMA, the NEAT chipset also added logic to support 1 MiB of RAM (or more) with a mapping of 640 KiB at 0..9FFFFh (classic PC address Range) and what's left starting at 100000h (above 1 MiB) into the ATs extended address space.
Timewise this happened ~1986.
Successive chipsets added more features. For example shadowing of BIOS ROMs (*2) and LIM-EMS support with the LEAP chipset. Of course other companies also started to build integrated chipsets. Ultimately this was the start (*3) of today's well known Northbridge design of modern PCs.
*1 - At least not while being compatible. Non or less compatible machines, like the SIEMENS PC-D did offer a continuous MiB or RAM.
*2 - Already at this time ROM and even more EPROM chips where much slower than RAM. Every BIOS access did slow the machine down. With shadowing the BIOS content got copied into RAM, with write protection acting as fast ROM emulation.
*3 - The SuperXT chipset for 8086 machines was released after the NEAT one, despite what the lower chip-id suggests.