You want to cull after fruit set for the reason you mentioned, pollination, and also because some of the fruits in a cluster will not be quite as strong as others. You won't be able to tell which flowers will set, and which ones are the highest quality, until after pollination.
Most Guava varieties are self pollinating, but you may get a bigger harvest with pollination between two varieties. Guavas are generally pollinated by bees, but for off season fruit, when the bees aren't active, you have to use hand-pollination.
Some large orchards thin flower rather than fruit, because pollination is very high. For a person with only a few plants, pollination rates are lower, and fruit thinning gives better results.
From here, a very interesting study showing the affects of thinning on guava production:
The study was carried out at the Germplasm Centre of Fruit Tree Improvement Programme (GPC-FTIP), Bangladesh Agricultural University, Mymensingh during the period February, 2011 to March, 2012 to find out the effect of shoot bending and fruit thinning on productivity of guava.The treatments of the experiment were four varieties of guava..., Swarupkathi, BAU
piyara-5, Chiang Mai (round) and Chiang Mai (long) and six different management practices..., (i) control (no shoot bending + no
fruit thinning); (ii) shoot bending; (iii) 25% fruit thinning; (iv) 50% fruit thinning; (v) 75% fruit thinning and (vi) 100% fruit thinning.
The ages of the plants were 4-5 years.
Different management practices had significant effect on all studied parameters in both season, 50%
fruit thinning treatment showed the highest fruit yield (20.46 kg/plant) and 75% fruit thinning treatment performed height TSS (14.73%)
during on-season but during off-season shoot bending performed highest fruit yield (13.50 kg/plant). In contrast, above studied
parameters were recorded the lowest in control plant where no shoot bending and no fruit thinning were applied. In case of combined
effect of variety and different management practices 50% fruit thinning treatment combined with Chiang Mai (round) produced highest
yield (23.15 kg/plant) and 75% fruit thinning perform height TSS (15.83%) during on-season, but shoot bending treatment combined
with Swarupkathi gave the highest yield (16.06 kg/plant) and 100% fruit thinning combined with Chiang Mai (round) showed highest
TSS (17.94%) during off-season
50% Fruit thinning as described below will dramatically increase yield:
Fruit thinning: Total number of fruits per plant in each
replication was counted after fruit set. Out of these
according to treatment fruits were thinned randomly by
hand when the average weight of individual fruit was 15-
20 g.
Shoot bending can help increase off season fruit production:
Shoot bending: The branches which were healthy and
disease free lateral shoots were selected for shoot bending
with care. Shoot bending was done in such a way that the
bent branch did not broken down after bending. Shoots
were bent at its antigrowth (at 90° angle) with the help of a
piece of rope. Before shoot bending 3-5 leaves were kept
at the upper portion of the branch to continue its
photosynthesis and respiration process and rest leaves
were removed off.
A rule of thumb from The Archives of The Rare Fruit Council of Australia
So here's a new 'rule of thumb' for the year of the guava: if you can touch two guavas with your thumb, they're too close (I favour the metric thumb).
From Sunset The Pacific Monthly:
While the fruits are usually about one to one and a half inches in diameter, they may be much improved in size by proper cultivation and careful thinning, as the guava is extremely productive and sets an enormous quantity of fruit. Indeed, it is claimed to exceed in productiveness almost any known fruit-tree.
Here is another good resource that involves thinning guava fruit.
