1. I use the TOD point as a cue to make my PA, usually 5 minutes before the TOD point and then request descent clearance around 1minute to TOD. If control/centre give me descent clearance at my discretion prior to that point then I wind the MCP ALT to the clearance flight level. If you enter the descent winds it gives quite an accurate descent point.
2. For the 777 there is a DES NOW key which gives approx. 1250ft/min rate of descent until you intercept your VNAV profile. It won't delete your altitude constraints on the leg page unless we press the ALT button on the MCP. Each subsequent push on the button deletes the next constraint, so if there are 3 constraints on the STAR then pressing the button three times will delete all the constraints. In practice it becomes a non issue because the flight profile is higher than the constraints and somewhere around 10,000 to 15,000 feet we go to basic mode (no LNAV or VNAV but HDG SEL and FLCH or V/S or whatever) so constraints wont matter.
3. STEP TO is useful as a guide for when to climb. Depending on the step size you enter into the FMC (RVSM or ICAO which equates to 2000ft or 4000ft) and the accuracy of the enroute winds from the flight plan which have been entered into the FMC and other factors (cost index etc), it will tell you when to go up. This should be roughly the same as your hardcopy flight plan. Normally I will follow the steps but I do check the optimum flight level on the VNAV cruise page and use that as a guide also. For the 777 the FMC will give you 3 levels 1. Optimal 2. Max and 3. Recommended.