When I finished with mixing and mastering, should I record my midi drum files to audio, or it's not necessery?
Lemme get this straight.

You have one or more midi tracks, which contain the drum part, and that midi track(s) are being fed to a sampler/virtual instrument which actually gets you the drum sounds.

If that's the case, rendering the thing to an audio track isn't gonna do anything, 'cause the thing is being "rendered" in real time in the exact same way.
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I would recommend you render it out. First reason is it saves a huge amount of processor power and ram that you would use to run the sampler. Secondly, if you render it out then it forces you to treat it more like a real kit.
If you were going to print your midi track to an audio track, the time to do it would have been in the mixing stage, so that you wouldn't be running too many live VST instruments and putting strain on your processor. Since you've already made it this far, there's no reason to change anything now.

Might not be a bad idea to print them to an audio track before backing up all of your work, in case you ever come back to work on this project in the distant future and you don't have the same sample library for some reason, though. I've found myself in that position and it's a pain.
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It's not going to change anything.

If things are getting glitchy when you render it or play it back (Because your computer can't handle all of your live VSTs playing at once), it might be a good idea to bounce them to audio.

You can do this by routing it so that it's recording your midi track (the source) to a new audio track (output). It should keep all of your other added plugins, which are usually the culprit in glitchy playback. You should do this for each individual midi track.

The idea being that it's easier for your computer to play/render stems than it is for it to have a bunch of live plugins/VSTs running.

If your computer's already handling all of your plugins/vsts like a champ, there's no need to do this. Although, getting in the habit of making stems is a good idea.
The midi track will always be available as part of your saved tracks but I agree that recording the actual instruments on their own tracks whether it's midi based drums, keyboards or anything else is a better idea. Once you have the actual audio you can treat the tracks like live instruments and add effects (compression, reverb etc.) and EQ much easier. I use midi drums a lot (Alesis SR-16) and when I record them I usually record about five tracks of drums on their own channels (kick, snare, toms, cymbals, hi hat) by muting everything except the kick and putting it on a channel then muting everything except the snare etc. This way you can add reverb, EQ or compression to the snare without affecting any other drums.