The epubcheck error message that you got was triggered by ids that start with a number in (X)HTML or NCX files.
Sigil generated book ids that start with a number in the .opf or .ncx files won't trigger that message.
For example, I just generated a new epub2 book that was assigned a hex value that starts with number and wasn't flagged.
content.opf
toc.ncx
If you still believe that Sigil generated ids cause epubcheck error messages, please provide step-for-step instructions that allow the developers to reproduce this issue.