90% of your cases have ATX/BTX standard motherboard standoffs.
90% of the non OEM motherboards have ATX/BTX standard mounting holes
It is when you start getting into the OEM's and the mini's that things become different.
Here is some good reading about form factor standards
http://www.formfactors.org/default.asp
Again you have to understand that companies like Dell and HP that manufacture their own computers do not always follow the form factor standards.