The design flow I'm using:
• Design circuit: Xschem + ngspice. Simulate until requirements are fulfilled using TT models.
• Verify for all process corners, voltages, device mismatch, temerpatures: Xschem, ngspice, home-made python scripts.
• Layout + LVS + DRC: Magic and netgen
• Post layout simulation: Again all process corners etc. (see above)
All in all it's a lot of work. I'm currently implementing a precision current source and a low noise / high bandwidth TIA and it takes monthes to get good results.