If you enter into a contract to buy a house, you pay a deposit, usually 10%. If you decide to withdraw after this, you lose your deposit. That is what deposits are all about, otherwise, what is the point?
If you are going to spend hours in consultation & planning time, it is not unreasonable to expect to be compensated. There is also the possibility that you have declined another wedding for that date on the grounds that you were committed to this one, hence you lose again.
You should always take a deposit at the time of booking the date and you should explain in writing your terms & conditions which can be anything you like.
I usually ask for 20% of the estimated total value, even if the final figure cannot be calculated at that stage. You will not know if someone is just using you to compare prices or as a standby if something goes wrong with another florist. If you obtain a deposit, it tends to verify commitment on the client side.
Of course there may be cases where you decide to refund a deposit in full or in part. That is up to you and it could be related to goodwill or retaining a good customer. If you lose a customer due to witholding a deposit then you will potentially lose more than the deposit in the long run.

I did have a case where a bride cancelled due to a massive falling out, but 12 months later met someone else and married very quickly, so in that case it was the right thing to do. You need to look at each case individually, but always have a policy to fall back on.