I usually place forms in iframes (i.e. order form, request assistance form, contact forms, ect.). Just the forms, I never place other contents or pages in iframes.

From a SEO point of view, would you exclude forms from being indexed/crawled by Google or not?

I mean my forms hardly ever contains keyword/keyphrases, moreover I obviously place empty title/meta description tags in pages shown in iframe to display forms, cause those titles are never displaied in browser title bar. So I'm wondering what's the point of letting Google index them?

Moreover I think these form pages might suck out PR from all other pages that are more valuable for SEO.

If your answer is "yes I would exclude them form indexing" would you simply use robots.txt to exclude them?

Thanks!