i use an app to help me tune my ukulele, but it isn't sensitive enough to get it all the way in tune.
if it's an app that means it was made well after the year 2000, in which case it is definitely accurate. but what may not be accurate is the frets/intonation of your instrument. sometimes you can get the open strings in tune but if you press the frets it won't be right. this is called bad intonation and it essentially means you need to have a bit of surgery on your instrument because it may be slightly bent, etc