Tighten them down to the proper spec, about 300 in/lbs or really f'ing tight, and then don't touch them until you need to remove them. I had the same "problem" when I installed one of my cranks but it's not really a problem. As I understand it, the crank actually works onto the taper after you install the cranks properly, taking some of the load off of the bolts, which makes the bolt feel loose if you try and check them after installation and riding. It hasn't been this way with all of my cranks but just one of them and I haven't had any problems with it except when I tried to tighten it down after the initial installation and I thought the bolts were coming loose.