So would you advise doing something like that or not?
Depends entirely on how much you trust the operators of the proxy servers, and also how the site you're visiting manages their sessions.
It's nuts to submit any creditcard info through a remote proxy, that's for sure.
But to really answer your question: NO. Simply because the entire pretext of your question is whether or not it's save to use a proxy to access myspace, you're insinuating that the network you are on prohibits you from getting there. (I mean -- really -- there is little other explanation for this situation)
Using a proxy to circumvent such blocks would violate either the acceptable network use policies at your place of employment (which could lead to you getting fired for circumventing the block and 'abusing' their internet connection), or trying to get past a filter put in place by a parent/guardian, which you shouldn't be trying to get around either.
So... No. Bad idea all around.