The most obvious and significant conifer species to be concerned about is Yew. Most yew is shrubby but I'm familiar with at least a couple individuals that are large enough to resemble a small hemlock tree if you're less familiar. Yew is extremely toxic. If you wish to forage conifer needles it's worth learning its characteristics, as it's not hard to identify once you do. Useful reference: http://aspcapro.org/sites/pro/files/zk_vetm0905_646_650.pdf
All the hemlocks (the tree kind, not the flowers) are safe. Further information- http://www.foodasmedicine.ca/2011/western-hemlock/ Link mostly discusses Western Hemlock but Eastern Hemlock is functionally identical aside from having a different set of folklore and somewhat thinner cambium.
The junipers and cedars are safe for most people in moderate amounts; that said, many of them do contain relatively powerful compounds. Eastern White Cedar (Thuja occidentalis) for example is one of the richest sources of thujone, which is a stimulant at low doses but at higher doses toxic to the nervous system and liver. Consequently some sources state that it is toxic, but this is not the whole story. Thujone is also found in culinary herbs like sage and rosemary and brewing herbs like wormwood. The dose is the question. Essential oil of wormwood contains more than enough thujone to kill you, but liqueurs made with wormwood benefit from their much lower dose. The levels available from White Cedar may indeed be risky for those pregnant or nursing, for example. For myself, on the other hand, there's no reason not to enjoy some cedar tea from time to time. Or to brew mead with it, as I like to do. This site contains a good discussion of thujone: http://www.thujone.info/index.html
More here: http://ec.europa.eu/food/fs/sc/scf/out162_en.pdf
More on junipers: http://www.eattheweeds.com/junipers/
More on Thuja genus cedars: http://livingafield.com/Plants_Thuja.htm
For regular consumption across a range of constitutions the true pines are the best, though. Almost all of them are quite benign, except perhaps (again) if you're pregnant, as they are reputed to be able to induce miscarriage (also probably dose-dependent, but who would want to mess around with a risk like that?) Ponderosa Pine, also called yellow pine, is the only one that I am aware of being potentially harmful, and this seems to be usually connected with pregnancy risks as well, but to a much greater degree, at least based on its well-known toxicity to cattle. Clearly there are plenty who use it, but if you have multiple pine species around it may be best to concentrate on the others. Couldn't hurt. White Pine in particular is pretty much unimpeachable. It's easy to positively identify by the needles, which are attached in clusters of 5. White pine is the only pine with this characteristic. A good primer on pine tea: http://davesgarden.com/guides/articles/view/3126/#b
Anyone further interested in the ethnobotany and chemistry of human use of pine products for nutrition will find this paper a treat: https://dspace.library.uvic.ca:8443//handle/1828/3248
Pine-needle tea is measured to have several times the vitamin C content of citrus fruits. Well worth it in my opinion!