Flembo,
I second what Rich said about not wanting it to fit too tightly, and about having a bit of bow in a rod. Wallace Gusler also mentioned that a few months ago on a post on here. But I wanted to add that it doesn't take much at all. If you lay the rod on a flat surface and roll it, you can kind of measure how much it rises in the middle. I have one that only has a bow of 1/8" over 36" and it still holds in place just fine. Even if I hold the rifle muzzle down and shake it the rod doesn't fall out. The rod in one antique I own is similar. If fits reasonably loosely, and it only has a bow of about 3/16" over 47", and it holds in place just fine, even if the rifle is held muzzle down and shaken.
Also note that rods will usually warp just a little over time.
Based on that type of experience, when making rods, I go ahead and straighten them, and fit them so that slide in and out without being tight. Then I expect them to warp just a little over time. The rods in the antiques I own all have mild warping. This is so even on one that has scorch marks from where it was once straightened. Of course, it could be argued that this was because these rods weren't properly sealed. But to that I can only reply that back in the day no rods were "properly" sealed. So I really think the plan back in the day was to have them fit loosely enough that binding wasn't a concern and mild warping wasn't an issue, and in fact mild warping provided some springiness to hold them in place. If they warped badly, they could be straightened again.
The rods I am describing are rifle length, in hickory.
