I'm not sure about 'ą' - some examples would handy, but if we are talking about differences due to regional accents then following rules would be perfectly fine. With 'ę' - how do you pronounce 'część' actually? Again, I think the worst that can happen normally would be to be judged as 'ą ę'* ;)
I think that in general Polish pronunciation is fairly 'regular' and with applying just a few rules you would be almost always OK. Obviously I haven't try to learn Polish as my second language.
* For non-Polish speakers - if someone is 'ą ę' it means that (among others) he/she tries to be overly 'correct' in pronunciation.
Ą is a bit more regular at that, so usually loses the n when at the end of a word and never heard without n when in the middle of a word, except in places like Warsaw, and other cities up north, where I was called out as gòralski (which is funny considering I'm Italian, I guess I'm learning polish wrong but convincingly enough)
So yeah, everything is easy, simple and obvious when you are a native speaker and I stand corrected :)
Sz/cz etc are just your regular dyphtongs, like English sh/ch/th etc - they are perfectly regular and phonetic. The only tricky one is rz, but it's also regular - there's a rule saying when you read it as ż and when you read it as sz depending on the preceding letter. And there's like 3 exceptions when you read it as separate letters - in words like "tarzan" :)
As for regional differences - all versions are correct usually. People in Kraków even accent on the first syllable and read "trzy" as "czy" :)