There are two ways to look at writing style: formality and clarity.
The formality of the writing, formatting, and imagery will depend on your customers. A professional party planner may promise to “keep the drinks flowing, the music thumping, and the dance floor full until the small hours”, with pictures of happy people having a great time. If you’re implementing accounting systems, people might have different expectations for how formal your writing is. Even when writing formally, let some personality come through. People buy from people, not companies, but especially in formal proposals, remember that many people may read the document, not just your primary contact.
Note that the formality is different from the clarity of the writing. It’s possible to have a relatively informal proposal with great, crisp writing. It’s also possible (probable, in my experience) for a very formal proposal to be full of obfuscated language and just plain terrible. Clarity is not negotiable. Don’t try to sound “professional” by using fancy words and jargon. Don’t write: “It is anticipated that co-location within client offices will be required on a daily basis at the start of the engagement.” Write: “We expect to work with your team at your office for the first 2 weeks of the project.” Clarity means you understand what you’re thinking and your prospect not only understands your writing, but understands that you understand the problem.
So— set your formality based on your personality, your company, and most importantly, the customer’s expectations. But always write clearly.