It’s better to keep your elbows up and out away from your sides because it gives you better leverage factors over the bike. Try this simple test to feel the difference. Sit on your bike and hold the handlebars with a low grip and low elbows. Then move your upper body back and forth as hard as you can, then do the same from side to side. Now grab the handlebars with a high over grip and high elbows. Perform the same two tests. Which way feels like you have more leverage over the bike? And keep in mind that factor multiplies when you stand up.