I never design an interface without checking with the developers I work with. Is this possible? What's the time frame? How hard would it be to iterate? Can we do this nifty slide and layering action?
Then check with the product team/manager - is it worth it? Do we have the time/resources for the extra level of 'coolness'?
Then check with the DB/Backend guys - would this flow work, these values, these sort orders, creating of these groups, how does it fit with legacy entries?
Then check with the test users with interactive mock ups/wireframes - does it fit together? Does it flow? Are there confusion anywhere in the sign up process? I see you are entering this ridiculously long user name that is going to break the design? No, don't go ther - back to the drawing board.
If you have OCD like me and your math is off (ie trying to retain a 1px border on both retina and non retina of a shape that consist of multiple strokes is just asking for trouble), you can spend ages tinkering. Luckily, and hopefully, non retina iOS will be phased out soon.
As you get better, you'll slowly learn to see ahead and anticipate all the moving parts but dang, it can be a tricky journey. Lots of learning to go for me.
ie: I've yet to come across a app that previews my mocks up to my iPhone nicely. ATM I'm using Box. Dropbox doesn't work cause they, for some reason, wants to destroy the file by compressing it while on a mobile device.