The HX850 is about a year old (it's discontinued now) and the HX950 hasn't even been out for an entire year (also discontinued). The previous versions were the HX820 and HX929. there aren't any direct replacements in the 2013 lineup for the 850/950.
Buying a warranty is up to you, but I would make sure to read the terms and conditions for all available plans before you buy. Square Trade is cheap, but repair costs take away from the overall value of your potential replacement. E.g. you spend $1500 on a TV, and you need a repair. Let's say the repair is $400. Maybe you need a second repair that's $200. Then by chance, you need a third repair. You're left with $900 total credit towards a repair/replacement since you've already used $600.
ht Panasonic TC-P60ZT60, Monitor Audio: Silver RX6, RX Centre, Radius 90HD; Martinlogan Dynamo 700, Marantz SR5006, PS3, Oppo BDP-103D
2ch Marantz PM8004, Sony BDP-S1000ES, JVC T-X3 tuner, Monitor Audio Silver RX1, REL T3, Apple TV, Peachtree Audio DAC•iT
lr Panasonic 50ST60, Sony BDP-S5100