Well, depends on what you mean exactly. Assuming you mean bottled bacteria, I wouldn't trust it. Some people argue about the merits of various bottled bacteria, but I personally am not overly convinced that much of any of it on the market today has any real value. A couple brands seem to have some positive reviews, but most of it seems pretty hit or miss (usually miss). If you did decide to go that route, I don't suggest just trusting it. I would still get a test kit and add some ammonia to make sure it is working before stocking.
If you mean beneficial bacteria from another, cycled tank, that is a good route to take. Again, you might want to still do a little testing, but I find this method more successful.
If the issue is simply that you want fish right away, you can always do a fish-in cycle. As long as you do it slowly and responsibly, there is no reason that any fish would get hurt.
EDIT: oops, I typed too slow. lol