Yes any resin spills inside the machine are absolutely critical.
Formlabs does have instructions for cleaning spills but in my experience, if resin spills on the main mirror, cleaning it is impossible and the mirror should be replaced. I’ve tried cleaning such spills and have only managed to damage the mirror further in the process.
At this point, I keep at least 2 spare main mirrors on hand (contact Formlabs support. If you are under warranty, they just send one out no charge). I will say that Mirror replacement is very easy and only takes about 20-30 min. I would, however, make a small alteration to their instructions. They say to remove the protective film only after installing the mirror and then clean it as needed. This is not ideal as the mirror is difficult to inspect and clean once in place. What I do is first remove the old mirror. Then, remove the film from the new mirror, handling with gloves and Pec pads. I and handle the mirror only by the edges with PEC pads. Inspect and very gently wipe the mirror with a fresh, dry PEC pad. I never use isopropyl as it only seems to leave streaks. Once I feel the new mirror is perfect, I install the new mirror and close up the unit. To keep things clean from there, I always keep the lid closed and a UNUSED , spare resin tank in place. This leaves very little chance for even dust getting into the compartment.
I do have a few suggestions regarding Filling. While Formlabs says it is safe to refill your tank mid build with the tank in place, I never do this. Unless it is unavoidable, I always remove the tank for filling.
First, I do whatever I can to keep my models UNDER 90ml (you can check in Preform) including supports. If the build is larger, I take it into Maya or ZBrush and split it up and add keys for assembly. This is not an easy solution and does require a fair amount of modeling experience. But it is the best way to avoid mid build filling.
Sometimes, you just cant prevent needing a mid build refill. This is what I do. I pause (quickly press the main button) the build which should raise the platform to the top of the unit. I open the lid and hold a sheet of 1mm styrene plastic (the sheet should be large enough to completely cover the compartment opening. You should be able to find it at any hobby shop, even Michael’s in the architectural aisle ) with one hand to prevent the build from dripping. With the other hand, I place the cover over the resin tank (it may be low or empty, but no reason to take the risk) and then slide it out of the dock. Once the tank is clear, I can rest the styrene sheet over the compartment opening, completely covering it. If the build drips, it ends up in the sheet of plastic and not in the compartment. I then fill the tank OUTSIDE of the unit as normal. I replace the lid on the tank (so it can’t spill during insertion), raise the sheet plastic and insert the tank. Once the tank is docked, I remove the tank lid, remove the sheet plastic, close the unit lid and DONE 
If, for some reason, I am not able to remove the tank, I pour some resin into a disposable plastic cup and use a 10ml nylon medical syringe (without a needle of course. Got some at walmart) with a PEC ready to cover the tip. It takes longer but there is little to no chance for spillage.
Once it is habit, there is no reason the ever have a spill inside your printer. For that matter, you shouldn’t have to do anything other than occasionally use a Giottos rocket air blaster to clear out what little dust makes it in.
