So, could you tell us why the rims used to be built up but are currently not built up? That would seem to be very pertinent information for us to answer your question, no?
If the wheel was unbuilt because spokes started breaking, this is more likely to happen b/c spokes are too loose (fatigue to the elbow with repetitive stress) and in this case the rim is probably okay unless it was ridden for awhile with the broken spoke.
If a rim has dents, side-to-side wobble or a vertical hop, these will be difficult to get out in the wheelbuilding process, and if you can get the rim straight, it will mean that spoke tension isn't optimized or equal.
An undamaged rim will build up just like a new rim, no problem. But it's true, as DOS says, that it can be difficult to tell if a rim has problems. At least you should do the flat-table check as San Rensho suggests. And also check for cracks around any of the eyelets, as Dos suggests. But if you're confident the rim isn't damaged, then go ahead with it.
One thing that may be worthwhile, for a rear wheel, it to build the wheel with the rim flipped. So normally, rims are built with the logo being readable while standing on the drive-side of hte bike. Do the opposite. Because the drive-side spokes are much higher tension.
This way, if there are any not-yet-visible cracking of the eyelets (which would nearly-always come on the drive side) they'll now be paired with lower-tension spokes.
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"c" is not a unit that measures tire width