Ulead DVD Movie Factory (DMF)/Ligos Encoder -- Settings question - I-Frame Only?

DMF uses the ligos encoder. It has several pre-defined settings including DVD 720x480 HIGH and LOW. The HIGH setting appears to be 6mbps VBR and the LOW appears to be 4mbps VBR. The low setting gives GREAT results for my VHS to DVD captures and results in about 2:20 per DVD. The HIGH setting gives me slightly under 2 hours. I want to try 5mbps. So I go to the CUSTOM settings. Everything is straightforward except one thing......

My question is: Under "custom settings" for encoding there is a check box for "I-frame only". It defaults to YES/CHECKED. Is this the proper setting for DVD MPEG2 encoding?

By the way, this DVD-MPEG2 encoding process is currently taking about 3:1 on my 1gig P3. I'm about to stick in a second processor (dual) and hopefully that will cut it down a little.