in soil, i found that it's better to water it in before i place any seeds in the soil strictly for the reason that sometimes i have found that if i do a heavy watering after i plant the seeds, they can either float away kinda or get really covered over with more soil than i would have liked to do originally be the big wave of water and soil that comes after a big watering..
saying that though, not too sure how much soiless mix is going to move around like soil does after a nice watering...
if you watered them before you planted them, you should be good for now depending on how big of a container that they are in... the trick is to put a finger into the medium, and if it feels dry down about an inch or so, water them till you get a good amount of run off out of the bottom of the pots.. before you water them when you think they need it, pick up the pots so you can get an idea of how heavy they feel when they get dry, this way after awhile, you can pretty much tell by the weight of the pots instead of having to get your finger all dirty, lol.. either way though, just the weight way takes a lil time to get a feel for them is all..