what if you end up in a bottom house?

Girls can go to any party they want. However "top" sororities generally hang out with "top" frats more. Hence, at a sigma chi party all of pi phi will probably be there and maybe a few people in a "lower house". It's not as big of a deal for girls, so I'd just go with the people you like most

Don't worry about it, you can still show up to any party you want. I'm in a "bottom" house and dated an alpha delt for a while. As long as you as an individual are cool people don't really care what house you're in.

The difference is mainly just reputation, top frats wont reach out to your house for exchanges but you can still have them, and there are tons of great guys in the lower tier houses too so honestly I'm super happy and have no regrets about where I ended up.

so even though it's against panhell rules top frats will have "closed" or "private" exchanges or pregames/prefunks with top sororities for normal parties. This is the only time it makes a really big difference. Like I can guarantee you that Sigma Chi will never have an exchange with aopii even though they're right next to each other. Another example, my friend was dating a beta and they were having a pregame with pi phi before a really big triad and she literally had to stay in his room and couldn't go to the party otherwise the girls would have treated her terribly.

People might say that these closed parties don't matter that much but lots of girls get asked on grab a dates and to formals during these parties so they matter if you're trying to go to any of those

You can get the same Greek experience in any house. Maybe if you value "closed" parties with "top" frats then you should join theta or something but honestly the guys in those houses are really douchey and look down on normal people. Not cool to hang out with in the first place.