Yeah, the parking after 6pm/on yellow lines thing is a great and little known dodge. It used to be even better as it was free after 12 noon on Saturday and all day Sunday but now I think the council have tweaked it so it is just after 6pm weekdays and Sat and all day Sunday.
The free parking runs all night through till 8am so it used to be a god-send when you had guests at a city centre apartment with no extra parking. If it was a three hour meter they could park after 6pm, load the meter so it clocked through till 11am the next day, and then have a relative lie-in before they had to move it. Most of the central meters do now seem to be one hour or two at most.
The council have also spoilt the fun by latterly removing a lot of the bays and changing single yellows for doubles (all part of the council's strategy to drive cars out of Manchester!) but i did note at the weekend that the row of bays round the corner from Piccolino (turn right at the top of Clarence St, alongside Lime) which had been shut for the construction of the (lovely) Chancery Place building have now been reopened.
As they've been shut for so long they seem to have dropped off people's radar as there were ten bays in a row empty on a Saturday lunchtime when everywhere else in town was stacked out. The Opera House would then be a five-ten minute saunter across Albert Square, and down Deansgate to Quay Street.
Cheers
Thom