So my question is do I need to check the "add drop table" everytime or will it be ok leaving it checked at "add if not exists" ??
Basically, NO....

There are instances where one might select a different set but they must really understand what they are doing

The DB while robust is fragile to being structured correctly....

If all you want to alter is the product description....

First make a backup, place your site down for maintenance,

Tehn make a backup/export of ONLY the product description table.

Preserve a unaltered copy of this table, open another copy in a code editor like notepad++

Do a replace and enter the current text exactly in the for box

Enter the exact text you want displayed in the with box and select replace all

Save and import this table to your DB

Take off maint and check that you have not broken things

If you have....import the unaltered back of that table and you should be back to where you were

All else fails, import the full DB backup you made before starting