ILD, sorry, I don't think I was clear or communicated the idea well. I wasn't proposing exiting a bull market. My plan is to buy and hold for decades, as is the gospel here. I meant buying only...no selling involved. For example, now I'd buy international until a US correction. Then, holding on to the international, switch to buying domestic during the length of the correction. Does that change your answer or is it still dubious market timing?
It does change my answer. It doesn't seem nearly as bad as I first imagined. Still pretty bad, though I'm not sure I'd call it market timing. It does seem like an arbitrary way to form an asset allocation without a plan, and with no particular goal in mind.
The approach still suffers, albeit in smaller increments, by avoiding putting new money to work in a rising market. So while you're not bleeding performance in gushes, you are still leaking performance in drips and drops that could add up to some pretty big numbers, in terms of lost opportunity, over time.
When your stache grows to a certain size, your periodic contributions will have less and less of a meaningful impact on your asset allocation as time goes by.
At some point you will retire and stop accumulating. At that point you will have to decide whether the asset allocation you ended up with is what you want to retire with, and what to do about it, if it isn't.