Some postdoc applications specifically mention who the principal investigator. However, other positions don't have a PI; the postdoc is being hired to join the research group in general and, while there's an expectation the candidate will collaborate with group members, there's nobody who is going to be the "boss." Typically these positions just say "the candidate will join the research group X of Professors A, B and C, working on topics such as..."
When applying to this second type of position, should I mention in my cover letter which professor(s) I'd like to work the most with? Something like "I am particularly keen to collaborate with Prof. B, as we share interests in..."
On one hand, doing this shows I'm personalizing the application to the position and that I put some effort thinking whether I'm suitable for it. On the other, it could be seen as being inflexible ("I want to work with Prof. B and nobody else!") and naming names might be perceived as tacky.