Yes it will happen. Because when you return true in the onclick event of the save button, it will cause the form to be posted back to the server.
So if you want to do some custom validations, you should go for asp:CustomValidator.
Lets suppose you've a textbox named "txt1" and you want to put two validation controls on it. One is a RequiredFieldValidator and other a CustomValidator. Our CustomValidator will check if the length of the text is greater than 5. If it is not, an error message will be shown.
<asp:TextBox ID="txt1" runat="server"></asp:TextBox>
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="txt1" ErrorMessage="Please enter text" ValidationGroup="VG"></asp:RequiredFieldValidator>
<asp:CustomValidator ID="CustomValidator1" runat="server" ControlToValidate="txt1" ClientValidationFunction="ValidateMe" ErrorMessage="Text length must not be less than 5." ValidationGroup="VG"></asp:CustomValidator>
<asp:Button ID="btn1" runat="server" Text="Submit" ValidationGroup="VG" CausesValidation="true" />
and the javascript method for doing the validation will be
<script type="text/javascript">
function ValidateMe(sender, args) {
if (args.Value.length < 5) {
args.IsValid = false;
return;
}
args.IsValid = true;
}
</script>
Please refer to following link for more details.
http://www.4guysfromrolla.com/articles/073102-1.aspx
Hope it helps.