A drill bit will always drill a channel with the top of the channel wider than the bottom of channel (swaft being pulling back up the drill bit flutes cause the cutting edge of the side of bit to hollow out more of the wall edge as its being wedge over..
Reamers are tip tapered, so you will drill a pilot channel under size first by around .015" (bottom will be .015" smaller, while the top of channel may be way less), then ream the channel afterwards to produce a true to size channel top to bottom. As, due to swaft, you want to peck ream, making to sure keep the swaft as minimal as possible in the channel/reamer flutes.
As for the channel, it will only be as true as the tool that you are using to hold and turn the tool. Even the best collect and tool spindles may still have some run out, but normally in the .001" range on a good mill set up*, so keep that is mind if you are trying to produce a channel down to the .001" range.
*To verify run out on the machine/tool/collet, indicate off the reamer shaft just below the collet to hold the reamer in the spindle. If you are using a chuck instead of a collet, then could be upwards of .010" or more, so what every the number is for run out, need to downsize the reamer, since it's the reamer size, plus the amount of run out at the tool level that will be dictate the channel end size.
Lastly, ream slow and keep in mind that if the reamer has any run out it as well (bent), it can increase the size of the channel as well (happens more in softer metal, but can happen in harder metals too).
To add,if you are working/holding dimensions down in the ten thousand of an inch range, your into aerospace range of precision machining, and good luck getting there with anything short of a million dollar set up (read just normal wear of a reamer from one cut can cause wear to the cutting edges to produce a defective product that puts you out of the ten thousand of an inch hold ).