I teach stretching classes for dancers (non-professional mostly) and our rule is stretch often. So generally speaking any time is great for stretching. We do contract our muscles a lot during the day, don’t we? Why not stretch them often? The important thing is WHAT KIND of stretching you have in mind.
I always do a minor stretch first thing in the morning, to wake me up and I recommend it to my adult students as well. We always discuss what particular stretching exercises are appropriate for a waking body. You wouldn’t do a full out stretching in the morning. But starting your day with a bit stretching is great for those who work on their flexibility and need to maintain a certain flexibility level. When you’ve been sitting at a desk for a while, it’s good to get up and stretch a little. Again we are not talking about any major stretching, but giving your muscles attention is always a good thing.
In order to do a good deep stretch, you need to be properly warmed up, I think everyone knows that. Stick to that rule and stretch often!