Just keep in mind allocate opens and closes twice. Right now, you have totally free reign to put in your preferences however you like. It then closes and their (im sure super powerful) computer sorts it to give everyone the best option possible. After it reopens, its in whats termed "adjustment mode" where you can basically swap your way into any classes that aren't yet full. Of course, at this point, all the good spots would probably be taken.
If you're lucky and stay on it all day, you might be able to pick up a good spot as someone swaps out of it (it works in real time). This is why its a good idea to check straight away, to make sure you got the spots you want (or at least close ones). If not, your best bet to try swap will be on that day since thats when everyone will be most active swapping in and out of classes. It might seem obvious to us oldies but these little tips can really help smooth it over for new students.