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.