How often does assimilation take place?

The one to speak is the one which is most clear and pronounces all of the letters. Regardless of which accent a native speaker uses on a daily basis, they will be able to understand English spoken as it appears in print. So if you pronounce the words individually, you will be understand. This is what I tell students when they say "I'm going to Ireland and want to sound like a native," or "Can you help me speak English the way they do in Italy so I will be understood?" Users here will vary on what they consider a standard accent, however "BBC English" is considered standard. I am American and speak with a "Broadcast Standard" accent. In my opinion, either of those two pronunciations can be understood worldwide.

Of the examples you gave, I would choose this: ʃi hæz uːzd ju.

Speakers from different areas will combine sounds and omit them based on habit in their area. If you want to imitate the speech from that area, you'll need to focus on one speech pattern and find a teacher to help you with that. So in my case when a student says they want to sound Irish, I advise them to find an Irish teacher who has the accent you want, because Ireland has many different accents.