Hi.

I'm reloading 6.5x55 swe, and made a ladder with Norma URP powder. The precision was, by far, the best when I had 1.5 grs under recommended minimum load. The brass have a lot of soot on them after shooting, and this is supposed to be a sign on low pressure. The brass is neck sized (fire formed in the same rifle), and i tried some variation in bullet hold, bullet depth and crimping (using crimp die -Lee), without significant change.

So my question is, why is the soot a problem, except for dirty brass and chamber? Should I offer the precision for some safety reason I'm not aware of?

It just means that the neck is not fully obturating and sealing off the gas; it is common and nothing to worry about. Annealing necks might stop it or it might not.
URP is a slow-medium powder so you are not getting high pressure anyway.
Higher pressure will usually stop most of it. Some pistol and revolver cartridges will soot up the case something fierce; like Starline brass in .45 Colt at Colt pressures; that will never seal the gas.
Just shoot.

That's good news. I got the impression that soot was a disaster, but never knew why.

And I do clean the chamber. And the brass. On occasions, I even clean my self.

I get some soot on almost everything I shoot and have for 60 years. I've always thought it was because I was a little lax in removing all the lube on the cases?????? roger

I shoot nothing but long range. I never use the full house loads as they shorten the life of the brass and wear the barrel faster. 3/4 loads have always shown to be the most accurate in any 6.5 plus caliber we shoot.
The neck of the cartridge almost always sits in a little oil from the last time the rifle was cleaned. This may prevent the neck from fully sealing the chamber and venting some gas back and showing "soot" on the neck. This will depend on the powder being used as well and its burn rate.
This is normal. Soot below the shoulder is not.
Annealing the brass may help stop some of the soot and going to a faster powder may as well if the soot is a problem for you.