Hi Leon,
definitely you need to use the correct type, unless you have a very good reason not to.
The decimal places for quantity and price is not controlled by you or your program.
It is a Company wide setting.
You can find the setting in Administration - System Initialization - General Setting under the Display Tab.
Note that whatever you change will affect the company, not only your add-on.
And also, you can increase decimal places, but you can not decrease the decimal places.
Regards
Edy