Without an IMA bypass (as you described), the DC-DC converter won't run properly. My brother is actually having the same issue with his - he's getting an IMA light sometimes, and when it's on, his 12v sometimes starts to drain because the DC-DC shuts off.
Here's a good guide to make sure your DC-DC converter stays on:
Hybrid Automotive - Honda Insight (2000-2006) Battery Bypass Instructions