So, I'm in the process of installing wood floors in another room in my house. It's the same wood, from the same sawmill, but it has been milled differently than the first batch I have (which turned out awesome).

The wood is 3/4" white birch, 3.5 inches wide. The first batch had two 1/8"x1/8" grooves down the center of the bottom for moisture. The wood came untrimmed, so we trimmed the ends on all of it, and then grooved them and when laying them down put them together with spline.

The second batch is the same, except the two grooves on the bottom have been replaced with a 3/16" deep channel that is about 1.75" wide. If I groove the ends of each board, the groove will intersect with the channel. This means that nothing will be underneath the spline, and the spline wouldn't be doing hardly anything.

I talked to the sawmill, and he said that he'd just put them end to end and not worry about the spline. He said he's had other customers do it that way and they haven't had any problems. However, I'm worried that over time, I might end up with height differences where the ends meet.

One thing I thought of was using a biscuit jointer and a thin biscuit at each end to ensure there was plenty of room for sanding in the future, and plenty of wood under the bicuit to hold it down properly (or at least better than nothing). Is this a bad idea?

What should I do? Should I attempt to fasten the ends to each other somehow (ala. biscuit jointer or something else), or should I just butt them up end to end and not worry about it?

Many custom milled floors are not end matched and most installers don't go to the trouble of adding splines and bisquits. You can do that, no harm. Do not glue the ends together with bisquits or splines however.

So you think it will be OK without biscuits or splines?

I should note that when we used spline on the first floor, we did use glue, but only on one side because we glued the spline in a couple of days before actually laying the floor.

Gary would most likely add, that as long as you did not glue the endjoints of the boards together, you would be ok. the spline, or biscuit, would help hold the endjoints "level' with one another, in the event of movement.
My apologies to Gary, If I mispoke in his place

I won't use anything, including biscuits if you think I can get away from it. The sawmill guy said that he had changed the way he did the moisture channel to reduce the chance of cupping. I think there were some problems with some of the installs done with his wood. I haven't noticed any issues on the original design, just some major gaps during the winter, which were remedied by installing a whole house humidifier in my furnance. It was 50% humidity when the wood was installed, and it dropped to 19% during the winter. Now I'm able to keep it at 35% during the winter without condensation on the windows, and it seems to have mostly taken care of the gapping issue.

The wood is only 9/16" thick in the middle where the channel is cut, so I would think this is more likely to cup, but I must be mistaken.

Yeah, you're right SK. That's what I was refering to. I was thinking perhaps he was adding and gluing the spline as he went along. Doing it that way could end up gluing the end butts together, which I don't think would allow for movement very well, if needed. BTW, glad to hear from you again. You doing OK?