Riding flats while learning fundamental techniques like the bunny-hop is very recommended. Using clipless while learning these can teach you to simply pull up on your feet rather than move your body on the bike, which can put you in a dangerous position if your front wheel were to get stuck. As you get into mountain biking though, it's mostly personal preference. I like flats for downhill and techy trails while I ride clipless for XC races. Going over a lot of roots when it gets rainy can throw your feet off flat pedals which is why I tend to race in them. Flat pedals will let you get off the bike quickly if you're about to take a tumble.