Regarding covering the pollinated hip:
On page 11 of the Summer 1994 issue of the Rose Hybridizing Newsletter, Dr. Leo Dionne states that he covers the pollinated hip for 2 reasons. The first is to prevent the rose seed chalcid from laying its eggs in the developing rose seeds. The second is to prevent Botrytis hip rot by preventing the spores from reaching the hips.
I would like to add a third reason: to prevent stray pollen from reaching the flower.
Title: POLLEN AND POLLINATION EXPERIMENTS 1. THE CONTRIBUTION OF STRAY POLLEN TO THE SEED SET OF DE PETALLED HAND POLLINATED FLOWERS OF APPLE MALUS
Authors: VISSER T; VERHAEGH J J
Authors affiliation: INST HORTICULTURAL PLANT BREEDING IVT , WAGENINGEN, NETH.
Published in: Euphytica, volumn 29, pages 379-384, (1980).
Abstract: " Using the finger nails to remove most of the petals is a fast method of preparing apple flowers [cultivars Cox's Orange Pippin, Elstar, Golden Delicious, James Grieve, Melrose and Smoothee] for hand pollination in the balloon stage. Such flowers still appeared quite attractive to insects; left to open pollination, the seed set per flower averaged 2/3 of that of similarly treated, but hand-pollinated flowers. With the aid of marker pollen of scab or mildew resistant donors and the subsequent screening of the seedling progenies for resistance, it was shown that the stray pollen was responsible on average for one of every 3 seeds formed by unbagged depetalled and hand-pollinated flowers."
As seen in the picture, after the hip has grown the bag is often sideways or downwards. This position allows rain water to run down the stem into the bag. I recommend cutting off the corners of the bag several days after pollination. If you are pollinating in very hot weather, you may find it beneficial to poke a lot of small holes in the bag. I poke the holes with the same tweezers that I use to remove pollen. I place the bags on a piece of styrofoam board and poke away.
I use a paint marking pen to write both the name of the pollen parent and the date of pollinating right on the plastic bag. Note, I use a paint marking pen (found in craft stores) not the more common ink marking pen.
Since my 2 inch by 2 inch clear plastic zip lock bags are also used as the label, I do not consider that they take any more time than making any other type of label.
The bag also makes it easy to see which hip was pollinated, and I assume that the bag discourages squirrels since I am not aware of squirrels bothering the covered hips.
As far as the concern that the bag is going to cause the sun to "cook" the hip, that is not my experience in zone 5 northern Ohio. Possibly, this is because the bags are clear (plus the fact that there are three open areas that allow air exchange - around the stem and the 2 cut corners. Of course several days after pollinating, the corner cuts can be made as large as is needed even to the point of cutting the whole top of the bag off. Higher temperatures are thought to be one method to overcome reluctant pollination so letting some heat build up right after pollination may be beneficial.