To the best of my knowledge, the U.S. Marshals Service does not routinely require applicants to submit to polygraph screening. Their employment FAQ makes no mention of any polygraph requirement:

http://www.usmarshals.gov/careers/faq.html

However, to address your questions in the event that you were to be extraordinarily required to submit to a pre-employment polygraph:

1) Yes, mentioning that you had handled these plants is precisely the kind of admission that can be spun out of all proportion and used to wrongly disqualify you. For example, it could be quite easily be mischaracterized in the polygrapher's report as: "Subject admitted to hands-on involvement in the illegal cultivation of marijuana." At a minimum, mentioning that you handled marijuana plants will raise a huge red flag and set off alarm bells.

2) No. While applicants for positions of public trust have an ethical obligation to answer relevant questions truthfully, they are not obligated to reveal every indiscretion or personally embarrassing incident.