A few minutes ago, I tried a fun test with a pair of KiwiSDRs, one of the W3HFU receivers and K1VL Vermont. Instead of looking for a zero beat, I had two browser tabs open and went to upper sideband mode, tuning 1000 Hz below CHU. I could then listen for the beat between two tones at 1000 Hz.
Without using the 1hz url parameter, I could not tune arbitrarily between the 10 hz increments on the K1VL receiver. It would round and tune to the nearest 10 Hz. With 1 Hz tuning enabled, I could tune in 1 Hz steps with ease. After having fun doing it by ear, I went to SAM mode and did indeed notice the audio panel's sync offset display respond to those 1 Hz steps (or not, if I didn't have 1 Hz tuning enabled).
That sync offset is a pretty slick way to compare receivers, if you tune them both to the same station! After picking and poking around on several receivers, checking then against WWV, I can say that most are within a couple of Hz and some are dead nuts on frequency - within a few tenths of a Hertz, plus or minus some QSB effects.