There's got to be some way to sugar-coat "I'm the one with 20 years of design experience, we should really do it this way".
In a professional, calm manner, explain standards and user expectations. Show the customer other sites with similar forms. If they still want the data entry fields centered, then do what they want while making sure they acknowledge it is against your advisement. Depending on your relationship with them you may want it in writing.In future projects you should have specs for each page that list the data fields, whether they are read-only or editable, fonts, justification, etc. You can also have a general spec for fields saying all fields are left justified unless otherwise specified. Also specify that change requests will probably cost them depending upon the nature of the change and how far along you are in the construction.
If the customer later wants to change the specs for a page then enforce your change request policy.
Above all else, always set the customer expectations. People understand that if they are building a house, it is cheapest to move the bathroom before the architect does any work; it is more expensive to move the bathroom after the architect has drawn all of the plans; it is much more expensive to move the bathroom after the house is framed out; and it is most expensive to move the bathroom after construction is done. Make your customer understand that it works the same way with website and software development.
