Is it a good idea to purchase your own professional liability insurance 2

Generally not needed. You should be covered by your employer's policy. It's best to get it writing of course, but most policies will cover all employees acting on behalf of the company. I interned with a firm that limited who could sign/seal documents to a select few, and I believe they were able to get a discount on insurance because they guaranteed to the insurance company that only those people (who all had 25+ years experience) were acting as EOR on all projects. So make sure your firm doesn't have anything like that going on.

It's also a good idea to be indemnified by your employer. That means they'll protect you (pay legal bills) if you're named in a law suit doing work for the firm.

My opinion: unless you're sharing in the profits (your wages are not profits), you should incur no financial risk as a result of the work you're doing. If you phone one in and somebody gets hurt - you deserve the fines/license revocation/prison time the criminal courts will hand down. But if there's a civil spat between your employer and a property owner over the bounce in the floor you designed, that should be on your employer since they're making all of the profit from you.

Many will tell you that the lawyers only go after the ones with money and that's why you shouldn't get a policy. There's probably some truth to that and certainly some anecdotal evidence to support it, but there's nothing stopping them from naming you on a suit and it doesn't cost much for a lawyer to tell his paralegal to add your name to the list. But as long as your employer indemnifies you (whether directly to you or indirectly through a clause in an employee handbook), you should be ok.

After all that - I'm not a lawyer! If you really want to understand your legal risks (and that's really what insurance is about - paying for lawyers and covering the cost of the suits people bring against you) you should talk to a lawyer about it.

Absolutely not.