Operational impact of filtering SMB/NETBIOS traffic?

Due to an increasing number of intrusions into windows-based machines through unprotected shares, I've started filtering both incoming and outgoing traffic for our customers on ports 138/139. So far this has caught a fair amount of traffic coming from customers, but none have called to complain about a lack of connectivity. Because this traffic is IP traffic, I wanted to ask others on this list how they treat SMB traffic on their backbones? Thanks -Scott