they do not need a suit for a checkout job or barwork...
I used to interview for checkout staff when I was a checkout manager at Sainsburys and I used to get a much better first impression from someone in shirt/tie (jacket to me was not that important) than trainers/jeans and we all know first impressions do matter.
For a proper job like a web designer I would definitely as a minimum wear shirt/tie/trousers and shoes.