Operational impact of filtering SMB/NETBIOS traffic?

[Scott Call <scall () devolution com>]

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