I initially thought to use interrupts as it is pretty critical that it happens regularly (I'm meant to be aiming for +/- 5 milli Kelvin stability), which is why I also thought I would need to check the temp just as often.
The target accuracy and PID timing and not necessarily intertwined. For example: If you're controlling the temperature of a 2 ton slab of iron in a well insulated room using the heating element from an electric kettle, sub-second updates are pointless. Any output change is going to take days or weeks to have a measurable affect. However, if you're controlling the temperature of the tip of a pushpin in a windtunnel using a 10KW heating element than sub-second updates are critical.
In other words, before adding the complexity of an interrupt service routine you need to first determine that PID timing really is that critical.