Because labor is typically the largest part of the job, I tend to replace everything with a kit. Really depends on how the car has been maintained.
On my 2004 V70R, the previous owner changed the oil every 5000 miles with mobil one.
That entire system was clean as a whistle.
On some of the other cars, that had been maintained by the book, the entire system needed to be replaced.
It looks like there’s about a $15 price difference between a set of six OEM Volvo, at about $15 apiece, or genuine Denso at about $12 apiece.
https://www.fcpeuro.com/products/volvo-spark-plug-s80-xc60-xc70-xc90-s60-31286359