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 60" ZT60, Monitor Audio: Silver RX6, RX Centre, RX1; Martinlogan Dynamo 700, Marantz SR5006, PS3, Oppo BDP-103D, Panamax M-5100PM
2ch Sony 32" W650A, Sonus faber Toy Monitor, REL T3, Marantz PM8004, Sony BDP-S1000ES, JVC T-X3 tuner, Apple TV, Peachtree Audio DAC•iT, Panamax MR4300
pc Monitor Audio Radius 90HD, Audioengine D1, FiiO A1
+ Sony 65" XBRX850B, Sony BDP-S5100