The PHH is right next to the EGR exhaust port and is also the first hose in the heater hose system, so the coolent is the hottest and I think at the highest pressure, when making contact with this hose.
I elected to change all of my hoses when doing the PHH. As it turned it, none of the others needed to changed, but it is something I won't have to worry about, probably ever again. The only other hoses I would be particularly concerned about are the hoses that connect directly the firewall. These are extremely difficult to reach and get too and would be far easier to work in the confort of your garage or driveway rather than on the side of the road in some emergency repair.
If doing other heater hoses, I HIGHLY recommend using the Toyota OEM hoses for the formed hose sections. I suggest using Toyota hoses cut to length for the other sections as well, except the PHH, which could be whatever you want, including the silicone hose. The reason I suggest the Toyota hoses for the straight pieces is that the larger and thicker hoses are very hard to get on the connections, while the Toyota hoses are easier and are still high quality hoses.
The hoses I did not replace where the rear heat hoses and lines, which are sold as a one piece kit. The outside diameter of the rubber part of this system is the same as the other heater hoses, but has a smaller inside diameter. I think this helps to make these probably the stronger heater hoses in the system and rarely need to be replaced. However, the metal lines, part of this system are painted steel and are located directly above the cat converters. Almost every single rig out there will have the paint flaking or flaked away. If you live in dry climates or non-salt corrosive regions of the country, the lack of paint probably won't be an issue. But otherwise, these lines are likely rusted right now and will eventually spring leaks.
I have some diagrams and information on my heater hose replacement page....
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