You'll need to shift up/down in the back if you're maxed either way. Suppose you have a 12-25 cassette:

if you are in say the 50/23 and need another gear, you can push it to 50/25 and have some front rub on the derailleur (maybe), or shift to the little ring 34/23. You'll go from a medium cadence to WOW SO MUCH SPIN. What you can do is a quick gear shift down to the 17t in the rear, then drop to the small ring, and be in the same ratio as the 50/25 with more gears to spare as the effort becomes harder, in a much smoother transition.


The opposite happens when you're maxing out the small ring. Going from 34/13 to 50/13 will have you hitting a wall of doom. upshift to say the 19t in the rear, then shift to the 50t ring, and it's smooth as silk.

Shift before you think you need it.