As someone who had a 2.4kg full-term bub (just below 3rd % and 100g underweight for a normal healthy baby) and spent almost a week in special care monitoring and with the problems small bub's can have (although she was otherwise healthy) I will be asking for growth scans in my third trimester of my next pregnancy. Only because I will be worrying about it constantly anyway, and am hoping for some reassurance of further advice as to what is causing it (I don't smoke) or how I can help grow bubs bigger.

If there is nothing they can tell you that will help, and you feel baby is doing well - then there's no reason for you to check. 20th % babies are still within the healthy range - there's all sorts of different sizes in healthy. Whatever will make you feel the most at ease is the right answer.