Of course it is. That's actually what I did both times I went dress shopping. I called and asked if they had the particular dresses I was looking for, and I also asked to make an appointment. Trust me it will make life much easier. The second time I went one of the dresses I called about ended up being the one. Its will worth it. Good Luck and have fun.
It is so ok to do that. I just hope you have better luck than I did. I found four dressed that I loved in pics and found out what stores sold them in my area. Out of 20 shops only 3 of them said they had those dresses in stock (each store had about 2 of them). My mom and I went shopping at all of the stores and guess what? NONE of those stores had ANY of the four dresses I had called about. I was so mad but instead of making a wasted trip I tried on some dresses and did find the one I think I'm going to get. So, just keep in mind that even if they tell you they have it they may be lying just to get you in the door.