It is your choice .... By "basic phase" of the project, do you mean the FEL1 phase ?
There is probably not one answer to this question that will suit ALL heat exchangers in ALL plant designs.
For preliminary plant layout purposes, the smaller units can be guessed .... Larger HXs should probably be sized using a detailed method
What kind of plant are you working on and how many HXs are involved ????
MJCronin
Sr. Process Engineer