Very few sales people mention equal site lines so the customer does not realise until too late that their new windows do not have them. It is not something that is generally considered unless someone mentions it and without knowing why, the brain understands that something is not right.
Opening windows are a frame within a frame. Equal site lines mean that a mock frame is put inside the non-opening part so that all of the lines are equal as in the picture. (Top right) Compare that to the picture below on the left and you can see the difference. It does not make a huge amount of difference but bear in mind that extra frame cuts out a little light.
Equal site lines cost extra but if you like things to look balanced, not having them may bother you. Either way, you should at least be given the choice. It is important that the purchase of windows is right as it is unlikely that you will be replacing them again for a long time.
If budget is really tight, you can choose to have them at the front of the house and not the sides or back.
If the sales man does not print out a picture of your windows, draw them out, so that you can properly see exactly what you are buying. Go to the trouble of printing a picture of your house and draw the windows on in marker pen so that you can see if you have balance and symmetry where possible.
Windows are expensive. It is worth taking the time to get them right. There is no comeback on the company because you will have signed the order.
For this and other tips on getting the order right and negotiating the best price, get the book. The Home Owners Guide to BUYING HOME IMPROVEMENTS.