For pre-flight planning, as previously mentioned, you require a valid forecast for your arrival time plus 60 minutes. This is achieved by having the TAF.
If your arrival is within the validity period of the TTF then that is what you determine your operational requirements on.
Of course if the end of the INTER/TEMPO with operational requirements coincides with the end of the validity period of the TTF then you have no indication of whether the operational requirements will cease at that time or continue into the next TTF issued while you are flying (as Capt Claret mentioned). There is the TAF but that is not as "accurate" a forecast as TTFs.
So if you have an arrival time of 0350 with a TTF valid for 0100-0400 with an operational requirement from 0300-0400 then what will you do if the TAF (valid for at least your arrival plus 60) says it's CAVOK?
I know what I'd do.
Edit: but that may not be the exam answer.