Hello.
Fall and winter is a common time to prune deciduous trees and maple. When pruned in late winter or early spring the wounds may flow with sap. This very much depends on the current state of (your) winter (still more or less dormant ?). If in (strong) growth already, pruning should best be delayed until midsummer as it can lead to disease. The other side is that, as you say, you know you are going to remove the wood anyway, why let the tree waste energy on it in the next months ?

If you decide to do it now then it is best to prune when you expect 2-3 days of dry weather so as to allow closing the wounds before rain.
Piet,

Here is a link that might be useful: Tsubo-en

I prune my Japanese Maples all through the year with no problems. A little leaking sap is no reason for concern.
Do you have enough room for your maple to grow without limiting the spread by pruning? If not, consider moving it.
They move rather easy if not in the ground for over twenty years and in decent soil.
When pruning, work from the bottom inside and go up, removing all dead branches and those that are being overlapped. You will end up with a thin umbrella. Do not stub off any branches or tips or expose the trunk to too much sun from the south. Stubs are the first sign of a beginner.
Here's one of mine.
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More can be seen at the address given on my homepage. Hit the search bar after entering Japanese Maples.

Here is a link that might be useful: My garden pictures