I always use underlay in filled designs, a denser one for things like towelling or fleece, a default one for cotton, etc. When the designs is meant for batiste, tulle, netting etc. I leave it out, except for sating stitch swirls which need a bit of body.
For (bought) designs without underlay I make sure the stabiliser is up to the task - rather a cutaway than a thin tearaway.

I agree with Mops. I don't digitize but I have noticed that designs that don't have underlay appear to be weak and I must use extra stabilizer to get a decent stitch-out otherwise the design seems to buckle. Listen to Mops!

I also agree with Mops...underlay unless very light or open weave design where you would be able to see the underlay under the open weave...

Martine said it all and I concur. :-)
hugs n roses, Meganne