Starting out I always found it best to keep at least 2 of each of the most useful mods like base damage, health or shields.
One that is upgraded as much as you can and the other non or minimally upgraded to initially put in new weapons and frames until you level them up enough to fit the more upgraded mod.
One that is upgraded as much as you can and the other non or minimally upgraded to initially put in new weapons and frames until you level them up enough to fit the more upgraded mod.