I want larger width tracks for power distribution/supplies.
Eg. But then if I want 3.3V to use 1mm instead of 0.250 then I cannot route to tight pin-pitch devices - as a single netlist implies a single track width.
Should I be using net-ties?
What do others do?
Neck down the traces at the pins.
Or do a plane and connect to the pins with vias.
Let me expand on this.
When you set up net classes with specific design rules (track width and spacing), you're setting up two things.
One is for routing. You tell the tool to just use the net class defaults and the traces you put down will follow the rules. This is handy, so you don't have to thinks about whether you've got the right settings for, say, a digital trace vs an analog trace vs a power trace. Just click and place your tracks.
The other thing is that the net class rules help ensure that you don't violate your fab's design rules regarding trace and space minimums. It's a good idea to always route with immediate design-rule checking enabled (which is the default) so you can't place a trace with a spacing to an adjacent trace that your fab won't build (or won't build for the price you want to pay).
Where this sorta falls down is in the situation you describe -- you want to place thicker power traces but you have to connect to a pad whose spacing to adjacent pads violates your net class rules. So you're stuck -- you can't set the net class rule to have a thicker trace because the DRC will fail if you need to connect to that pad. Thus you have to set the net class rules for the worst case -- the pad -- and remember to set the trace width to a larger value if desired when you're routing elsewhere.
(There are, of course, other reasons why one would set design rules, like to meet space requirements for high-voltage traces.)