Am I right in thinking breasts should be too high and square in the beginning for a nice outcome?

Yes, generally speaking,I think you are correct: breasts tend to soften and "settle" to some degree over the course of the first several months after breast reduction surgery. Exactly how much the breasts will change however will differ depending on many factors. One of these factors includes exactly what procedure is performed and the skill level (and sense of artistry) of your plastic surgeon.My best advice: select your plastic surgeon very carefully. Make sure that he/she has significant experience achieving the types of outcomes you will be pleased with. Then, communicate your goals carefully, as well.   I find that the use of goal photographs and computer imaging are very helpful during this communication phase. Working together you will come up with a good plan to achieve your goals.   
Best wishes.

Dear CrinkleBear,Thank you for your question. After a breast lift or breast reduction, results tend to "settle" and shift as the body heals. To what degree this occurs depends on many factors, including differences in each individual's body, skin and tissue quality, how extensive the procedure was and very importantly: the technique your surgeon used. I tend to advise my patients that, the way they look the day after surgery is not how they will look 3 months after. The breasts will be higher on the chest wall initially, before the breasts and nipple positions settle into place. 
In the end, each patient's results will be unique to their body. To get an accurate idea of what kind of results to expect, make sure you consult with a board-certified plastic surgeon thoroughly before proceeding with a breast lift or reduction. Hope this helps!

Hi and welcome to our forum!
The performance of breast reduction is an insult to the soft tissues of the breast, resulting in significant swelling which will initially distort and totally mask the result. The swelling may pull on the incisions, responsible for the initial "boxy" look at the inferior poles. The amount of swelling differs from individual to individual. 60-80% of the swelling abates in the first 6 weeks, with the remainder slowly subsiding over 6-12 months. One cannot determine the final result until the swelling subsides. Persistence of a boxy look may require revisional surgery at one year. 15% of cosmetic procedures require a secondary touchup. 
Although the surgeon is responsible for the operative result, the quality of the skin and soft tissues may play a role for any recurrent sag or unnatural look after the procedure. 
Visit a board certified plastic surgeon with considerable breast reduction experience for in-person evaluation and discussion. . Look at the before and after pictures.
Best wishes...