Sub-contractors and supplier lien rights

I'm a general contracting who uses sub-contractors on projects. We are not always aware of the suppliers they use to pick up or drop materials on project sites. Generally their accounts with suppliers are net 30 day accounts. In most cases completion of work and their invoices for payment are received prior to their suppliers requiring payment from them. In instances where they don't or can't provide us with a material release, is it good practice to hold their payment for 20 from their last day of work? Do their suppliers continue to have lien rights? If not what is the best way to manage this. Thank you Diane