Andy,
WELCOME TO THE AWS WELDING FORUM!!
That could be a rather loaded question.
Technically, yes, you should have E&O Insurance. But, define "some". For just a couple certifications per month I wouldn't worry too much about it.
But, are you going to do other inspections? That is where you will really open yourself up to needing the protection. Though you do have a certain level of responsibility that should not be taken lightly even in just doing 'some' certifications.
The rates I got ranged from just over $5000/yr to over $11,000/yr. And, I have heard of guys who say they only pay about $1200-1500/yr.
Most agencies that will carry business insurance can also link you up with E&O.
Have a Great Day, Brent