I have been adding a consistent amount of loose Corn sugar (5oz) to each 5 gallon batch. After my final fermentation just before bottling or kegging didn't matter and seems to work pretty well. The last batch I tried a premeasured hard sugar added to each bottle before capping. It also worked well. I wonder about the additions being sterile enough to avoid bacterial contamination. Actually any positive or negative experiences with candied sugar additions would be helpful before I change my routine for bottling.
It is quite possible to infect beer with non sterile priming sugar, although it is relatively rare for such an infection to occur. Mainly because correctly stored and handled sugar is relatively aseptic. One good way to go is to pasteurise the sugar by dissolving in a minimal amount of boiling water and adding to the whole brew (with stirring) in a racking bin before bottling. This also has the advantage that the sugar is well dispersed in the brew.
I have heard of paranoid brewers who microwave their sugar before use. Not sure if that is effective or not although it should work for sugars such as glucose/dextrose hydrates as the sugar crystals form with "trapped water".
However carbonation drops, which are quite widely used to prime beer in the bottle, don't seem to cause great problems in normal use so it is debatable if this effort is worth it.