tagged vs. untagged VLAN

[Jay Nakamura <zeusdadog () gmail com>]

In a SP environment, you need to hand off two VLANs to a customer, is
there any advantage or disadvantage in doing the following two setups?

- One untagged and one tagged VLAN
- Two tagged VLAN and no untagged VLAN

I can't think of anything other than some equipment may not let you
have no untagged VLAN.  But it's bugging me that something could go
wrong by not having untagged native VLAN that I can't think of.