Should I buffer my sensor before feeding it into an op amp or does the op amp buffer it for me
Stay close to DC and this is a question of output impedance of your sensor.

If you use a simple inverting op-amp configuration like this, you will have R1 as load for your sensor output. Suppose you have R1=1K/R2=10K and 100R output impedance of your sensor. That would not give you 10x gain but more like 10/1.1 ~ 9x.



You can have a much higher impedance as "load" to your sensor by using a simple non-inverting configuration. That may be what you want to do in cases where your sensor has a fairly high output impedance.

See more in this question: Mic preamp: Inverting or non-inverting op-amp configuration? 

I think you can answer the question simply by looking at the sensor output impedance (which may be expressed as some current/voltage at some load in the datasheet, but you get the idea).