If you can't remember the way back, and if you get truly lost, you don't need to know your way back to the exact spot you entered in order to get to the surface. You can just dig up (and fill it back in if it bothers you afterwards).
Even if you look at your current coordinates and the ones where you entered, that might not always help you (at least in a quick manner) if you're lost in a cave which may wander various ways and not directly lead you to that spot. So it's still often quicker to just dig up if you're actually lost.
I don't think I've ever noted coordinates of a cave I've entered.
I've never had any issues navigating caves either; at the most, I may place cobblestone to mark where I entered an intersection, or between the pillars in mineshaft intersections (this also blocks off any mobs). When I do need to return I go to the highest point nearby and dig a staircase (if I do not plan to return I just dig straight up from the highest point nearby and pillar up below me) to the surface and mark if with a pillar and record the coordinates if I plan to not return to it for a long time (otherwise, I can just look at the map I carry with me hand fill in while caving to see where I need to go to get back to it, or my current base. Even in the case where I lose the coordinates for old "return points" I can just look around the edges of the previously mapped area until I come across one, as I recently did when exploring from an area I last explored around 7 years ago).
I also explore so far within a single session, and spend several sessions at a time underground before returning, that marking the entrance is meaningless anyway (on average I explore about 100 chunks or 160x160 blocks per play session but rarely in such a compact area, I've gone as far as 500 blocks in a single session, much less caving trip):
This was done over 10 play sessions, and around 3 "caving sessions" with my double chest ender chest mod, 5 or so with a vanilla ender chest; each day represents the equivalent of around 75 tunnels, 2 ravines and a mineshaft (on average), explored over about 3 1/2 hours, with more than 3,000 ores extracted (I like to say that I do more caving in a single session than most players ever do in a world, which is not far from the truth).
This is a cropped rendering of the underground before and after; the area I explored between the two is about 700 blocks across:
always take a screenshot of the exit to a cave
NamePerson was_taken
Why?
If you can't remember the way back, and if you get truly lost, you don't need to know your way back to the exact spot you entered in order to get to the surface. You can just dig up (and fill it back in if it bothers you afterwards).
Even if you look at your current coordinates and the ones where you entered, that might not always help you (at least in a quick manner) if you're lost in a cave which may wander various ways and not directly lead you to that spot. So it's still often quicker to just dig up if you're actually lost.
I don't think I've ever noted coordinates of a cave I've entered.
I've never had any issues navigating caves either; at the most, I may place cobblestone to mark where I entered an intersection, or between the pillars in mineshaft intersections (this also blocks off any mobs). When I do need to return I go to the highest point nearby and dig a staircase (if I do not plan to return I just dig straight up from the highest point nearby and pillar up below me) to the surface and mark if with a pillar and record the coordinates if I plan to not return to it for a long time (otherwise, I can just look at the map I carry with me hand fill in while caving to see where I need to go to get back to it, or my current base. Even in the case where I lose the coordinates for old "return points" I can just look around the edges of the previously mapped area until I come across one, as I recently did when exploring from an area I last explored around 7 years ago).
I also explore so far within a single session, and spend several sessions at a time underground before returning, that marking the entrance is meaningless anyway (on average I explore about 100 chunks or 160x160 blocks per play session but rarely in such a compact area, I've gone as far as 500 blocks in a single session, much less caving trip):
This is a cropped rendering of the underground before and after; the area I explored between the two is about 700 blocks across:
TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.
TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.
Why do I still play in 1.6.4?