My opinion is that the rebuys are not usually cost effective, but the addon is almost always cost effective. Usually the addon is double or more chips than the rebuy will supply, and around the time you are allowed to addon, the addon is around 10 Big Blinds or more, which is a very significant increase to your stack. If you have over 40 Big Blinds you can start to feel secure enough not to buy an addon, but another 10 BB's can always be helpful in propelling you deeper into the tourney.
The argument could be made that rebuying immediately after your first blind is taken out can make a big difference, and it can, but you have to weigh that against your cashing frequency and the amount of variance you are willing to handle. The stack to blind ratio at the beginning of the tourney usually gives you enough room to maneuver without needing that extra depth, but if someone else at the table has already rebought up to double or doubled up through play, if you think you can beat them fair and square, it could be worth it to make sure you have an equal chipstack to take advantage of any opportunity you have when in a hand with them.