Until recently I have always watered seedlings from above, until I saw a few accounts on Instagram I follow watering from below.
The reasoning for watering from below is to encourage the roots to grow deeper, (as of course the roots are what takes up the water for the plant), and therefore they grow stronger.
I still water my seeds until they emerge from the top with a fine spraying bottle. As I don't want (especially small seeds) to be drawn further down in the soil.
To butt chug, I put my seedlings in this long container I bought from my local garden centre, and I fill it about half way up with water and leave them to take up the water for about half an hour. It is very easy to forget about them as I do quite regularly, so I now set an alarm on my phone to remind me lol.
I recently done a wee experiment, watering some from the top, some from below. It was interesting when it came to pricking out, I thought the ones I watered from the top were well watered, but they were far from it, the soil from the bottom quarter was bone dry!
I'd be interested to hear how everyone else waters their seedlings