Yes, you can wear an 18K gold wedding band against a 14K, 10K or 9K engagement ring. Although the 18K alloy is slightly more durable than that of the lower grade alloys, these lower grade gold alloys are still quite durable and compatible for everyday wear, and they generally won't react adversely to each other.
However, as 18K gold has a higher gold content than the other alloys and tends to have a slightly richer or different hue compared to the 14K, 10K or 9K gold alloys in the various colours. This colour difference may be noticeable when the rings are worn together. Over time, the 18K gold alloy will stand the test of time better than the lower grade alloys. However, this difference is usually minimal with normal wear.