This really depends on which engine your talking about. If it's the B21F then the head may need some valve repair before the bottom end of the engine needs any repair. The later B230F is a low friction design, and it is absoultely true that if there's any wear in either the valve train or the bottom end, that the entire block and head get re-built together. Doing them separately is a waste of time and does give rise to bottom end blow-by if the valves only are done. This mechanic knows what he's talking about.
The reason why this topic rarely comes up is that the later B230F wears evenly for valves and rings and when it needs complete rebuild is around 300k miles under normal service. I have been involved with Volvo's for 30 yrs and the B21/230 for a few re-builds and never had heard of anybody doing the head re-build separately from the bottom end on the B230, but the B21F is not so.
In almost every case, as mentioned below, the valves themselves can be serviced, but are rarely replaced on either engine.
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'89 245 Sportwagon, '04 V70 2.5T Sportwagon