In Architecture Wizard, the interface allows you to customize the buffers for a DCM to use "lowskewlines". The HDL for the USELOWSKEWLINES attribute gets written incorrectly, which causes a warning in XST and causes the constraint to be ignored. For example, the generated HDL contains:
attribute USELOWSKEWLINES : string ;
attribute USELOWSKEWLINES of CLK90_OUT : signal is "";
As a result of the "yes" missing from between the quotation marks, synthesis will generate the following warning:
WARNING:Xst:2403 - Value of property "USELOWSKEWLINES" elaborates to a null string. The property will be ignored.
It is true that the HDL generated by the Architecture Wizard is incorrect, but in fact the Use Low Skew Lines constraint is not valid for any architecture for which you can use the Architecture Wizard to generate a DCM; so even if the HDL was written correctly, the constraint would be ignored.
See the entry for the Use Low Skew Lines constraint in the Constraints Guide:
and you will find that it is supported only on Virtex, Virtex-E, Spartan-II and Spartan-IIE devices.
Therefore, you should not be using this option for your custom buffers in the Architecture Wizard.