The problem is that the exact frequency is much harder to predict than the time of arrival on the summit. I have learned from experience that it is much better to search long for a free slot on 40 m, if necessary in the entire band, than to use 30m or 20m unless I have bagged at least four QSOs.
So it is really hard to tell upfront - I typically alert as 7.031-cw, but might be forced to escape contest traffic and will end up at 7.002-cw.
On 30m, less people have good antennas (as the recent survey has shown), on 20m condx are unpredictable, and on both bands, the first skip is often too long to reach the many chasers in Central Europe from a summit in the alps.
73 de Martin, DK3IT