Let us first remember that the title of the article is "How to Be a Successful Club or Bar Cover Band" which does not apply to many of us here, although the 'Rules' could be applied ... that said, 'Rules are meant to be broken' ... or at least 'bent' to fit the need ... and many of these 'rules', while basically a good guide, need to be bent to suit the occasion ...
When we had the band in NY, we would have a 'set list' for background dinner music - 75% bossas, 25% soft swing or ballad instrumentals ... once the dancing began, the crowd dictated what songs were going to be played ...
We also put on a 'show', with our lead vocalist being the main attraction ... generally it would consist of a medley of songs following some thread - current holiday, certain entertainer, etc. ... we had the lights, props when necessary, the whole bit ...

These days, I prepare a set list for just about all the NH/AL gigs, and again, they usually follow a theme centered around a holiday, a certain songwriter or entertainment personality, or music genre - Big Band, 50's, Beatles, etc.
In NH/AL situations, my 'show' is my performance, because more often than not the audience is sitting down, watching and listening ... Obviously, a lot of interaction with the audience is needed here ...

Playing restaurants requires reading what the audience is doing - often early in the evening they are eating and having conversation, while later on it might be another cocktail and some dancing ...

Playing private parties, I will always get input from the client as to what they are looking for and what the 'demographic' of their guests might be ...

As far as 'set times', when we were playing weddings (back in the day) the standard was 40 on/20 off or 45/15 ... however, how many times did it happen that we would start a set and 10 minutes into it the catering hall manager would come over and say "we are serving the soup, stop playing..."
So much for 'set times' ...
To me the only time that is 'set' is the start time, which for for me is at least 5 minutes earlier than what was designated - unless instructed otherwise by the client ...

The mode of dress - while considerably more casual is accepted than in the past - always depends on the function and the venue ... I am not going to play an outside b-b-q party in a tux ... nor am I ever going to be mistaken for one of the maintenance staff ...

I always do a sound check, using either an mp3 that has a good range of highs and lows, or a quick record on the kb of a vocal, play it back and go out to hear what the audience will hear ...

As I said, the list is good as a base, but it needs to be adjusted to fit the occasion ...

Last weekend, while visiting family in NY we went to a local bar/restaurant to hear a band our son had heard previously ... they were 4 guys in their mid 60's - except for the drummer who was younger - and they played late 60's' through 90's rock ... they obviously used set lists because they went from one tune into another with barely a beat in between ... and the set lists worked because they knew the audience and what they wanted ... and they ROCKED the joint ...