Personally, I mix the drums first and then apply gates where necessary. Placement is certainly helpful, but no matter what mics you use and how you place them, sometimes gates are key to the sound you are after.
First, I usually end up with gates on a kick drum. This gate is not used to block bleed form other mics, but to help contain the resopnance of the drum itself so it is not "humming" when not being played.
Second, I rarely gate snare drums. Usually the snare drum contains too much energy from other drums and when gated, you can hear your drum mix change every time it is opened and closed.
Third, toms usually end up with some sort of gate. Arthur reccomended taking all of the sections between hits and manually reducing them. This seems pretyt silly with a DAW since most any decent gate will do this as well without the tedious repetition and unantural gain changing that a manual edit creates. Too mnay people think of a gate as being a mute and unmute device. They can be set up like this, but that is just the tip of the iceburg with any decent gate. Most gates have a control that you can set to manipulate just how much gain reduction the gate is apllying when the trigger is closed. You can set them form just a couple of db of gain reduction, or 15 db, or infinite etc... By using this in conjunction with attack and release settings you can make the volume sweels sound much more natural and minimize how much the single channel gate is affecting the cymbals, snare etc... of your whole mix. You can also use these three settings with your threshold to change the "timbre" of the tom shot. You can set it with a very fast attack, fairly high threshold, a lot of reduction and a pretty quick decay to get a very aggressive attack on the toms with not much decay (often works well with heavier drum sounds). You could also try a lower threshold (more natural) with a bit of a slower attack and longer release and about 6 to 12 db of gain reduction to get a smoother more rounded tom sound that has a lot of decay and less emphasis on the stick shot, and more emphasis on the resonance. This works great with slower stuff. A big part of the reason that I often end up gating toms is the same as the kick drum.... to minimize the volume of the "shell wobble" in the mix. Take a listen to your tom track solo'ed sometime. Notice how every time the kick and often even the snare is hit that the tom itself will start to produce a note. This is almost like a small false trigger of the tom itself. The physical vibration of playing certain drums (especially when played hard enough) often vibrates the other drums and heads enough for them to resonate their shells and produce a small note. When mixing big rock drums this can often result in a chorus of mush that is very distracting. With some styles though a little of this is desirable, depending on the drummer, the style of music, how well the drums are tuned, and just how that extra ring fits the mix. I see it as being similar to string squeaks on an acoustic. Often times it is something desirable, but quite often it may be a little much and need some sort of reduction.
