When you don’t give a project the proper effort, often the results are not satisfactory for you or your clients. Then you find yourself in the position that you have to go back and do it again. Not doing a task “right the first time” often creates twice the amount of work.
There are times when mistakes are made and these need to be fixed, but there are also times where jobs are rushed simply so you can move onto the next task.
If you don't have time for the task you are doing now then there is little chance you are going to have time to go back and fix it if it isn't right.
There is nothing wrong telling a client "I just want to get this perfect for you so it may take me a little more time".