Is this your own personal experience or the experience of someone else, about which you have read in books or online? I ask this because I myself own both a IIsi and a IIfx ROM. I have used both extensively in more than one SE/30 motherboard. And I can tell you right now that the IIsi ROM does not in any way produce any kind of artifacts at startup. No horizontal lines, no checkerboard patterns. Now, the IIfx does produce horizontal lines for a few seconds at cold boot, but I have never, ever seen a checkerboard pattern result from the IIfx ROM!
For additional information, you can read my two AppleFritter Posts here.