Based on the number of people of who had issues upgrading to 7.4 due to independent upgrades of these core packages, I would only recommend it to people who truly understand what they are doing.
I believe these issues were related to installing non-centos produced rpms, not rpms released BY centos. I agree with you that there are howtos out there for installing php5 on SME that could cause upgrade issues.
However, the packages contained in centosplus are built to respect the core release and are updated within 72 hours of a core release:
http://mirror.centos.org/centos/4/centosplus/Readme.txtSME 7.x updates should not have an issue with centosplus packages as any new centos released packages, which SME is based on, will respect the dependencies of the centosplus packages and visa-versa.
While centosplus packages are not officially supported by SME, I feel that the php and mysql updates can be made with very little risk to the end user.