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The idea is that you want sufficient exposure to get the boring featureless background above the noise floor. If you can get that above the noise floor then any signal is going to be more than sufficiently exposed.
I personally image with the same exposure as it means a smaller Master Dark library and it just makes it easy to run the calculations for total integration. As long as you know that for each filter the exposure is long enough that a dark featureless background is sufficiently above read noise, then it doesn't matter whether it is a strong Ha emitter (red emission) or a strong reflection nebula (blue emission), it'll always be above the read noise.
Although I've never done planetary, I'd imagine that most of the time you'll be about half filling the wells where the planet is in frame so I'd suggest that unless you've got a VERY dark planet (frame rate too fast), you're going to be well above read noise.
I also don't often hear of anyone using calibration frames when doing planetary or luna imaging.
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