I once had a new client tell me I was the first grantwriter who hadn't yelled at her and lost my cool at some point during the process. She wanted me to know she really appreciated it, and she's become a regular customer.
While clients won't use a grantwriter who can't bring in the awards, they'll also shy away from one that is tempermental, disorganized or difficult to deal with. But when the deadline is staring us in the face, we grantwriters don't always know we're being difficult. We're just getting the job done.
So I've found it's wise to ask for feedback from my clients.
About once a quarter I put together a simple on-line survey asking them to rate my service and comment on what they like and what can be improved. (I use www.surveymonkey.com -- it's free for surveys under 100 responses.) Sometimes I get surprised. It's always instructive. And when the comments are positive, it feels really good.
So don't be shy. Ask. And don't just ask the boss. Whether you freelance or have a position on staff, it's a good habit to periodically survey all the people who work with you preparing applications and proposals.