I use a router planer setup to make all my boards flat. Just putting that on here so every body understands why I ask what I do... As for the question, I have a project in progress which has screws coming in from the bottom (sanded flat with the wood). I want to make it flat for which some boards are a little higher than others. For the screws I am worried that if I use the wrong bit I will wreck a nice router bit, and risk my safety. What would be recommended for this (what can handle about 2 dozen screws spaced out). I have Carbide tipped straight, and solid carbide CNC router bit. If these are a bad choice what should I get, HSS bits? Note: This thing is 4' by 2' 4".
I don't think there are any woodworking router bits that were designed to handle metal screws in the wood being cut. Carbide bits will certainly cut the screws (as carbide is much harder than the steel in the screws) but the screws will definitely chip the cutters on the bits. HSS probably won't fair all that well.
You might consider trying metalworking CNC bits, though I'm not sure about the quality of the finish on the wood with these bits. Also make sure to use the correct speed on the router for these bits.
Alternatively, the best approach would be to remove all screws, process the wood and then screw it back. If you need the screws to hold the shape of the wood, consider clamping (if possible with the orientation of the cut) or gluing with hot-melt glue/double sided tape.
A final thought is to countersink the screws more deeply, so that the router bit won't touch the screws.
If you can get screws made of some very soft metal (aluminium or softer) then you may be able to do it, machining very slowly, skimming off maybe 0.2mm per hit... but on the whole it's a bad idea. If the screws (or remnants of screws) come loose at all during machining then you're going ruin the router bit, probably ruin the workpiece, and have a dangerous chunk of metal flying at you at high speed.
You need a rethink.