No — the kernel buffers non-O_DIRECT writes to block devices to ensure correctness.

Larger writes will be more efficient, however, if only due to reduced system call overhead.

While not necessary when writing an image with the correct block size for the target device, even partial block overwrites work fine:

  # yes | head -c 512 > foo
  # losetup /dev/loop0 foo
  # echo 'Ham and jam and Spam a lot.' | dd bs=5 of=/dev/loop0
  5+1 records in
  5+1 records out
  28 bytes copied, 0.000481667 s, 58.1 kB/s
  # hexdump -C /dev/loop0
  00000000  48 61 6d 20 61 6e 64 20  6a 61 6d 20 61 6e 64 20  |Ham and jam and |
  00000010  53 70 61 6d 20 61 20 6c  6f 74 2e 0a 79 0a 79 0a  |Spam a lot..y.y.|
  00000020  79 0a 79 0a 79 0a 79 0a  79 0a 79 0a 79 0a 79 0a  |y.y.y.y.y.y.y.y.|
  *
  00000200

Partial block overwrites may (= will, unless the block to be overwritten is in the kernel's buffer cache) require a read/modify/write operation, but this is transparent to the application.

Finally, note that this applies to most block devices, but tape devices work differently: partial overwrites are not supported, and, in variable block mode, the size of individual write calls determines the resulting tape block sizes.