Dear Harry,
You should the standard time types but strongly recommend to don't touch any of them.
First of all you should specify your grouping for time types. Then make a copy of all records of V_T550A which has group 01 into your grouping, for example 99. For grouping you should customize two most important views during your implementation: V_503_ALL and V_001P_ALL
This is the most important note because if you use the standard time types you will be able to use almost reports and SAP standard PCRs. But don't touch any standards to make sure won't loos anything by upgrading and so on.
You can add your own time types into V_T550A also and you will need some new based on your requirements for example for count some thing or for quota generation.
As you know any changes in V_T550A is part of you customizing and you have to create a request for this issue. So you can transfer your request to other clients without any problem.
Regards,
Omid