I only do quad and glute stretches off the bike (20 second each pose). NEVER use the backrest of your chair when sitting in front of your computer or working.
For back stretches, I do it entirely on the bike. Begin the first five minutes of the ride in small gears, minimal resistance, riding SLOWLY while hands on the drops with elbows bent. Relaxed, not tensed in that position. You shouldn't feel any pain, soreness, feeling overstretched when doing it. If you do, back it off a bit. Pedal at high rpm cadence but otherwise legs relaxed and at low power output level (recovery level effort).
I did some yoga before but found the above to be better for me.