Always, Always take a deposit and always get a written contract
Just to come at it from the other perspective...
I'm a customer. I agree the price and never quibble when it comes to paying, unless the job is carried out badly and you have refused to rectify. I'll expect you to be able to provide reliable references, and if you want, I will put you in touch with other trades and suppliers I've used so you can check whether I'm fair and prompt when it comes to paying up.
When I ask for extras I expect to have to pay for them (but if the tradesman throws them in that's OK too). I'll be fair to you if you're fair to me. But I will NEVER pay a deposit to secure your services, sorry. If you ask for one, I'll go elsewhere.
I'll be happy to sign a contract when I engage your services, but I won't pay a deposit unless you have to undergo significant, non-refundable expenses before you show up on Day 1. (I will be happy to pay a percentage of the fee on Day 1 of the job)
Furthermore, if i want to charge me for a quote (rare, but some people used to do this during the boom) I'll tell you I don't pay for giving you a free sales lead/walking into your shop.