Nutrient temperature swings

Anyone have any opinions on what would be acceptable nutrient temperature swings in a RDWC systems? For instance, how much would a gradual 3 degree swing in a 8 hour period stress plants assuming temps remain somewhere between say 67 to 73 degrees?

None. The impact of raising the temp will be for dissolved oxygen to drop so that would tend to reduce growth. The drop in DO is measurable but the practical impact of the change shouldn't be significant.

At 73°, the chances of root rot go up. If you're using bennies or running sterile, it won't be an issue though.