I like keeping my shoulders healthy so I would not do upright rows under any circumstances.
However, to answer your question in general, if you are losing good form, your weight is too heavy for the targeted reps at your particular strength level and you are asking for an injury.
Seeing that you are doing upright rows already, I'm not going to say either way whether or not it's smart to do them with heavy loads. That aside, I don't understand your reasoning, unless you are not using respectable form for 8 reps either? It sounds as if you are attempting to increase the load too much over what you are using for 8 reps. Add 0.5kg/1lb to the bar over what you use for 8 reps. How many can you do with that? Rinse and repeat until you find what your 5RM is without changing your form. Maybe it'll only be 2kg more than your 8RM.