windows 2008 r2 sp1 guest VM -- safe to add/remove vCPU??

We are running ESXi 5.0 U2.

Is it safe on a Windows 2008 R2 SP1 guest to add and remove vCPU??

The current VM has 1 vCPU and we want to consider adding 1 additional vCPU during payroll weeks when it does a great deal more number crunching than usual.

There must be a way to add/remove vCPU with PowerShell??

Thank you, Tom