More is better for z axis travel but also adds complexity. If you are going to cut out parts probably 2 or 3 inches is enough.
The problem comes when you have longs bits. If you have 3" of travel and your bit is 3" long you won't be able to lift the bit over a 3/4" plywood. if you design the machine to hold a 3" bit 1" over the table and you have 3" travel, if you put a wee bit in that sticks out of the collet 1/4" you won't get it all the way down to the table for engraving.
That said, most bits are not 3" long. 1 or 2" is common and tho so maybe 2" of travel is on the small side.
Eric
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