plants can only use 18 hours of light a day, they waste the rest. having said that, it cant hurt them.
also, the first few days is all about root production, so having the extra light could help.
If you just planted your seeds they are buried in the dirt hopefully greminating. No light needed until they sprout. 18/6 is what i recommend from my experiences. No need to keep them blasted for 24/7....and it makes less of an impact when you switch over to 12/12. You definately need to figure out the times and think about things like, heat, your schedule, vacations, visitors etc... This grow is going to take at least 3 or more months. Talk your way all the way thru it before starting the cycle.