In the poem If—, does the speaker warn his reader to be careful in spite of ideals? Give two examples from the poem to highlight this.

I think the speaker does warn the reader to be careful concerning ideals in the poem's second stanza, yes. He describes the value of being someone who "can dream—and not make dreams your master." In other words, he does not want to see people become slaves to their dreams: dreams and ideals are wonderful, but it is also important for us to be flexible, and if we will not settle for anything less than our ideal, we can actually miss out on lots of good things.
There's a saying: don't let the perfect become the enemy of good. It means that when we keep striving for perfection, we aren't happy unless we reach it, even if what we do have is pretty great. This line seems to endorse that same sentiment. The next line extols the virtue of being the type of person who "can think—and not make thoughts your aim." Again, the ideal is not the point; the point is experiences and living. We must be careful not to make the ideal, either in hypothetical or in terms of our goals, our master or else we could lose out on a lot of other great (if not ideal) things.