Work on whatever you can to keep progress up. But yes, ABSOLUTELY tie down or weigh down the tail. I had my tail installed when I mounted the engine, but took it off to get a little more working room in the shop. There was one day when I leaned forward while standing in the cockpit and threw the CG off just enough for the tail to come up and take me on my first plane ride. Fortunately, a buddy was right there helping me and grabbed onto the fuselage while the tail was heading skyward. He kept the tail from slamming into the shop ceiling--or slamming back down onto the rear stand. Either would have been bad. Very bad.