I’ve been an advocate for strength training to improve power and endurance since my comeback to racing a few years ago. I saw huge improvements in my climbing and endurance. The papers and books I’ve read also shows that if you are male 35 years or older, strength training is even more important because as you get older, you don’t retain muscle as the younger group does. Same goes for female cyclist, but age 30 year or older.
My training plans include work in the gym. During base and transition periods, strength sessions two days a week, while build and peak periods have 1 session a week. I also do not include strength sessions, other then Core work during “A” race weeks.
A good time to start a strength session is during the off season. Start with some adaptation, then work your way up to maintenance during the race season. I take a break in mid Oct, after my last XC race, then back on them in mid Nov. I don’t take too much of a break, because I don’t want to start over in the strength plan, but build on the strength I created over the previous season.
Here is a great article from Velonew on this topic.