I bag soon after the buds appear, and certainly before you see the yellow petals. After that, it's getting late. Or if the truss is still too small, wait till it gets to a more manageable size and nip off any open blossoms before you bag. Anyway, you can take the bag off as soon as fruit has set - little biddy tommies should be visible. Often, not all the blossoms have set, so I nip these off leaving only those that I know self-pollinated in the bag. I tie a piece of bright ribbon to mark the truss. You have to check fairly regularly, adjusting for growth and trimming leaves that get in the way. The bag should be big enought to leave plenty of room for growth, so that the blossoms don't get twisted etc. A trick I picked up from Earl on the US forum is to take a marking pen with you at picking time to write the name ON THE TOMATO. It's so easy to mix them up. BTW, leave these ones to ripen as long as possible on the vine so that you'll have nice plump seeds.

Ditto all of the above. Especially good advice to tie something to the bagged fruit (after you've removed the bag) to identify it later. It's amazing how quickly you can lose track of unmarked bagged fruit as the plant grows.

I actually nip off all foliage around the truss I'm going to bag. Only the truss goes into the bag with no possibility of further foliage growth inside the bag. I probably sacrifice a few growing tips, but I figure there are plenty more on the plant. I like to guard against the possibility of foliage growing and squashing the flower if I forget to check it.

And, if some sort of disaster with the bagged fruit arises, you can still save viable seed from a tomato that has just started to ripen. In dire emergencies, I've saved viable seed from tommys that haven't yet started to colour. As long as they've turned from dark green to that paler green.

Me too. This year was such a disaster that plucked a few green fruit in gabs. They had just turned, losing there chlorophyl, and I let them ripen indoors. Should be okay. Maybe germination will be down, but I'll get something!

When we drove through a high desert plateau in Mexico, we had a break and I went for a walk. Suddenly I noticed a scrawny (20cm high...) tomato bush with 2 small fruit - dark green and rock-hard. I picked one and carried it around for the next 2 weeks but it never softened or changed colour. I extracted the ripest-looking seeds (which were still whitish and soft) and bagged them after drying.
No, I hadn't "discovered" any new variety or another Lycopersicon sp. All the seeds sprouted and they were Roma tomatoes - similar to the ones sold at the local markets out of cardboard boxes labelled "Roma Tomatoes washed and waxed"...
So, even the most immature tomato seeds can germinate!
Rose-Marie

The quest for life is truly remarkable.