Almost always. Assuming that you’ve pre-qualified the client, in my opinion it’s usually worth the effort. I find that I have a much better chance of landing the job with a quality proposal. However, writing it does take time, so before you start, make sure that your services are within the client’s budget.
I also recommend using formal proposals for repeat clients. Never assume that your existing clients aren’t checking out your competition. A proposal gives you the opportunity to remind them and their management why they’ve chosen you in the past and why they should choose you again.
There are certain situations where I usually choose to use a more informal approach.
One of those is when the profit on a project is minimal. When it’s a super small profit, I may not take the time to craft a formal proposal. In those situations I will usually discuss the pricing with the client, and then send an email with that information. The only exception to this is when a client has much more work for us down the road, and they’re testing out our services with a small project first.