Yes. It is a good idea to establish company-wide limits for gifts so that there is no chance of undue influence, real or perceived. Limits in terms of monetary value are common, usually settling in around $40 or less on average for any one gift. That being said many companies ban the accepting of gifts entirely.
If you choose to set limited be sure to let your employees AND the vendors know of your policy as soon as possible. You may turn something innocent and intended to express genuine appreciation for your patronage into a nightmare of awkward conversations and hurt feelings.