Averil, I haven't been able to find out what date they are back to, when adding extra details to IOM marriage records in the LDS pilot project, but I checked a few Maughold ones there.
The 1850 marriage of Cath Looney & John Corkill has no parents' details on the pilot site, although Maughold par reg had these from mid-1849 onwards. Similarly with Isabella Kermeen & Robert Quayle marr. 1860.
I then tried the 1870 marriage of Isabella Radcliffe & Robert Christian - no parents' names on the pilot site, nor for Henry Kinrade & Esther Quayle (LDS "Mayle") in 1880. So it looks as though they have started adding parents only after the start of civil registration - I'll check this theory tomorrow.

Sue