I mainly stand up home and away, however it often depends what the people around you are doing and what stand you are in. For example, at Stamford Bridge if you're in the MHL, you will be standing up for the duration of the 90. If you're in the shed it'll be a combination of both usually, and on the sides if you stand up on a regular basis without any significant events happening on the pitch, you'll eventually end up getting kicked out.
In these modern all-seater stadiums, crowd control is much better with everyone being assigned a certain seat or in the case of a free seating arrangement, they only sell a certain amount of tickets before hand. The stands are no longer holding people who are stuffed together like sardines, so I don't see standing up causing a problem if everyone is assigned their own individual seat.
Standing is a far more enjoyable experience, does create a better atmosphere and at the end of the day, you are at a football match, not in your living room!