I would say no. It's not wrong, but it's the result of mashing words together when speaking quickly rather than following rules on pronunciation - overdoing it will make you sound slurred.
You'll never get into trouble by pronouncing words and syllables cleanly, but if someone came up to me and said "Ja hah'shockret." I'd wonder if perhaps they'd also had a bit much to drink. :)
It depends on which form the word originally is like ett hus / en mus -> when the ett/ en is before the word it is a/ an like in English. When that article is put to the end of the word, the word is in definite form, similar to the -> huset/ the house musen/ the mouse. And these are things you just have to know. In some words it changes to other comibinations - like blomman (the flower).
It's usually fine but I'm afraid it's not always the case. I've done some ~70 re-recordings which you'll find among comments now and then, but it's hard to say how much is wrong as a general number.
Again, though, it's mostly fine. You'll learn much more from having it on than you'll gain by turning it off. :)