I'm curious why people don't just use water?
People do use water, as Jafro shows in this epic video, but he also qualifies his statement in the video description:
I'm using tap water for the test because both cylinder heads I'm testing will receive extensive machine work and cleaning before being re-used. If you were to do this test on a freshly-machined head, you'd want to use deionized water as it contains none of the salts (sodium, chlorine, etc...) that would leave deposits and corrode metal parts.
So water can leave behind deposits or result in corrosion. Given you have just lapped the valves, I'd avoid using straight water for peace of mind.
And is a little leak like 1 drop every few minutes acceptable?
Or should it have no visible leak for x amount of time?
The video explains this quite well. You're best off waiting for something like 30-40 minutes in order to see if the meniscus has dropped. You should also inspect the inlet and exhaust ports for droplets to help pinpoint the source of the leak(s).