Precedents and slippery slopes can be powerful forces and can sound like powerful arguments. We should indeed think about the indirect consequences of our actions, not just the immediate ones. But the phrases "what about the precedent?" and "where do you stop?" don't magically mandate inaction. Rather, people who make these arguments must concretely explain how our action today would supposedly help lead to others' action tomorrow. They haven't done so here.
Our invading Iraq will not set a dangerous precedent or much of a precedent at all. We should focus on the costs and benefits of this war, and not on its supposed precedential effects on future wars.