The subcontractor can come after you for a bill paid to the contractor. Hence, it is typically advisable to get a lien waiver from the subcontractor whether or not the contractor also will be providing a lien waiver. Without a lien waiver from both, there is cause to worry. For more, I welcome you to consult the following source: http://www.life123.com/career-money/real-estate/lien/lien-waiver.shtml
And, if you would like to discuss with me personally, I encourage you to call me at 262-789-2741. Thank you.