You don’t have much choice in the matter if you want the stablising spick to go into the bottom of the trunk. I cut it straight across and have good luck with the trees.
I suppose the ? is with a scroll or coping saw, but you wont see your fancy cut in the stand anyhow. 
Can you graft a cut christmas tree onto some living stump?
I’ve thought of trying it on a seedling. You can’t graft wood with a very large cross section. I believe the maximum diameter that you can graft is about 1/4 inch You can of course do bud grafts to a larger diameter host. Once again it’s the outer layer just under the bark that will make or break a graft.
I don’t have the straight dope on warm, or cold. However, a tree lifts moisture up the trunk by evaporation from the foliage, needles in this case. Water molecules are strongly polarized and they line up positive end to negative end. The attraction is strong enough so that when molecules evaporate from the foliage other molecules in the chain extending down the trunk are lifted up the trunk to replace them. The passages are small enough that turbulence doesn’t disrupt the electrical molecular bond. It works all the way to the top of a 300’ redwood. Whether it works with dead trees or not I don’t know.
I’m like you. We always provided water for the tree and they always dried out.
That’s not a particularly close approximation of a tree though - most of the fluids are transported in the tissues just below the bark; a piece of dry timber doesn’t have that - furthermore, the process of drying it not only empties out the transport vessels (breaking their ‘prime’), but changes their structure too.
Nonetheless, water will ‘wick’ up a piece of softwood from the end grain to quite a considerable degree simply because of its fibrous, absorbent structure - we just shouldn’t assume that this is a result of exactly the same processes that occur in a living tree.
We always cut our tree the day after Thanksgiving at a local tree farm. I put it in a bucket of water outside till we’re ready to set it up (usually a couple weeks later) then cut the bottom inch off and stick it in the stand. I’ve had trees in the stand looking and feeling alive on the back porch till February. We always get a Fraser fir they seem to last longer than the more common Douglas fir that start dropping needles within a week.
We got a fresh cut Monterrey Pine on Tuesday. Two weeks before Christmas is about the limit, IMHO. It has been very thirsty. In the 40 minutes it took to drive home, the sap had already hardened on the base, so you MUST cut it again immediately before putting it in the stand.
Be very vigilant with water the first few days, checking several times a day. If it runs dry, you are going to have to make another cut. Not a fun prospect if you have begun to decorate. I let the tree sit for a couple of days before decorating to let it recover from the trauma and let the branches spread a little. My trees always last until after New Year’s.
At least you keep it on the back porch :rolleyes:. People, you’ve got to get those holiday decorations down by the first week in January so you can start decorating for our next major holiday, Super Sunday. Get with the program!
The only holidays I give credit are Christmas and The Forth of July. Luckly I don’t wait for one holiday to take down the other decorations. As you can see I’m not limited to only two weeks, which doesn’t give you enough reason to ever get out the decorations. Thanksgiving until the second week of January for my Christmas decorations.