Will be in Barcelona January 19-24. Necessary to pre-reserve for Sagrada Familia, La Pedrera and other sites at that time of year?
Probably not, but it might be wise to pre-reserve Sagrada Familia at least a day or so in advance so you get the time you want. Weekends might be pretty busy too even that time of year. La Pedrera's lines were about 30 minutes when I was there in early summer, so unlikely any issue there.
We have tourists all year around, when it's not summer vacationers, it's retirees in autumn, cruise parties in winter, students in february.... etc. So it's best always to pre-book for the most popular attractions so you don't have to queue in the event that particular day you find a bunch of tourists disembarking (some cruisers have 5000 people all wanting to go to some of these sites all at once, same day, same time!) you have your ticket! Surely for S.F. and Park Güell at least. For the rest, in January I would "risk" it.