As long its organic, most fruit and vegetable skin is safe to juice. However there are a few exceptions to this rule. As a general rule the peel form citrus fruits such as oranges, grapefruits, lemons and limes should not be juiced as they contain d-limonene which is classified as a toxin and can be harmful if consumed in large quantities. However the white pithy part of the peel can safely be juiced.
It is also a good idea to peal any fruits that have been imported from third world countries before you juice them. Often times health standards in these countries do not prohibit the use of toxic chemicals and pesticides which you do not want to be consuming.
Many seeds are safe to juice but there are a few exceptions. Apple seeds contain small amounts of cyanide and should not be juiced most pits such as plum pits, cheery pits, peach pits etc should not be juiced. It is fine to juice watermelon seeds, grape seeds and citrus seeds.
If you are unsure about the safety of juicing a specific seed it is best to look it up or just remove the seed to be safe.