Usually you can tell before something is complete if it is going to look good or not.
Doing a dry run of plant placement is the norm and always a good thing to do,set out the plants per design and make sure it will work.
I almost always have artistic freedom when it comes to my landscape installations,but sometimes that can be a bad thing too,best to do your expressing on paper 1st.When using materials and design a customer has picked out or made up their mind on can be tricky,when you Know it's going to look bad,you gotta tell them that,if they insist and you do it anyway it makes your work look bad and reflects on your reputation,
sometimes it's best to walk away from a potential nightmare mess that a customer has designed to save yourself embarrassment,
but that is not always possible either so you do your best with it and that's all you can do.
Most customers are going to like whatever you do to their bare dirt yard cuz anything will look better than what they have been looking at,
but it is up to us as professionals to set ourselves a standard of quality and not walk away from something that has turned out to look lousy from how we envisioned in the design.The cust is gonna like it no matter what 99% of the time.
Like olderthan said,just changing the placement of a few plants may make all the difference in the world to your design .
I never walk away thinking I don't like the way things turned out,I will change it or fix it to my standard of what I think looks good and on my dime.
I have walked away from bad customer design jobs,
and lost money doing so,but in the long run this has worked out for reputation wise.
What I have lost in money from those jobs I have gained back in pride and reputation