"just" is fine with the past simple.
People often say that it's American English - but you'll also find it used very frequently in British English - click
here for some British English examples of
just+saw, for example.
Remember that the past simple expresses an event that you want to see as past and finished :
A : Where's John? I need to speak to him.
B : Too late - he just left for New York.
Here, the speaker sees the event (John leaving) as being over - finished.
The present perfect, on the other hand, expresses an event which in some way continues into or impacts on the present. So :
A : Where's John? I need to speak to him.
B : I've just seen him in Anne's office.
In British English at least (American anyone??) B might choose the present perfect in this case to imply that if A moves fast, John will still be there and she'll find him - in other words the past event (seeing John in Paul's office two minutes ago) has a result in the present (there's a good chance that he's in there now).
But this isn't a "rule" - it's a matter of speaker choice. If you want to see the event as past (and this, I believe, is the tendency in American English) you use the past simple. If you want to see it as impacting on the present (common but not essential in British English) you use the present perfect. grammar is a means of expressing meaning. What meaning you want to express is your choice - so you choose the structure to express the meaning that you want to.