As long as the areas in which you are making changes are within the editable regions of your template, you should be ok.
for example within the content div...
<div id="content"><!-- InstanceBeginEditable name="content" -->
Lorum ipsum etc
<!-- InstanceEndEditable -->
</div>
Also ensure that the head section of your page is editable too.
I occasionally get the error message you refer to, even when there is nothing wrong.
When you refer to your validation, i presume you want the fields which have been correctly completed to retain their values.
there are a couple of ways to do this, either by passing the post variables back into the form, or creating a session variable for each field you want to retain the value, and passing these values back into the form.
in dreamweaver, in your text field properties, set the init val using one of these methods.
hope this helps
