Always. Always, always, always.
Because how you sketch your speech bubbles may not be how you lay them out later. You might find that the text you put in needs to be rearranged in a certain way either due to its length or how it structures itself line-per-line, or the speech bubbles don't work as is with the composition of the art ... and if you screw it up, you're gonna have to draw more stuff anyway to fill in those blanks you created by not drawing everything in the first place.
It also can't hurt to have untouched art with no bubbles to reuse for promotional reasons, thumbnails, memes, etc. without worrying about the speech bubbles getting in the way of it or the art being unfinished when you do take those bubbles away. I say this confidently as someone who used to cut corners by not drawing the art 'behind the speech bubbles' and 9 times out of 10 it bit me in the ass later.
Art first, lettering later. You can obviously still plan out where you're going to put your speech bubbles, because speech bubbles/text play a huge part in how you create a composition anyway; you gotta make sure the art and speech bubbles work together... but it can't be at the expense of one or the other. At least when you have the panels finished 100% regardless of their placement, you'll have more freedom to move your bubbles and text around without risk; and you'll have a bigger batch of art to pull from if you need to use old panels for whatever purpose.
And if you're someone who works on a team of artists, your letterer reserves the right to beat you upside the head if you provide them art that's not finished lmaooo