Roses are one of the most beautiful ways to decorate a cake. If you can pipe a rose with frosting, theres a lot you can do on a cake. I have made many cakes in the past and used roses on them. I will explain a few methods of adding roses on a cake to make a gorgeous cake.
1- You can add roses just all around, going with the other designs. For example in the first image cake I piped designs in the corners and put roses along the rest of the border.
2- Just putting a bunch of roses on the side of a clean white cake makes a clean neat look. Especially if you only use one other color. This technique also looks best with roses made out of fondant. It is important to add another design (usually writing) in the center of your cake.
3- A bouqet of roses is the most common way of putting roses on a cake. If you go take wilton cake decorating lessons, that is the first thing they teach you to do with a piped rose. Adding leaves, a bow, and stems perfects the look. This method is something that will never fail to look good. If you have like 15 minutes to add a decoration to the cake, this is what you do! (Thats what happened in my third cake)
4- A spray. This is the hardest way but I think its a very beautiful way to add roses to a cake. You put 3 roses in the lower center of the cake, and then you make stems and leaves coming out of it and rose buds on the ends (you can also add roses with a smaller tip and not completely done to make not fully opened roses).
Note: The last two cakes are some of the first cakes I have ever made (as a part of Wilton cake decorating course 1!) so they do not look that beautiful, but these were the best pictures I had to show you the examples. If you master these methods, though, your cakes will look very beautiful!