There is ALWAYS uncertainty when you take experimental measurements.
You will definitely want to report your elastic modulus, yield strength, and ultimate tensile strength with an uncertainty (the tensile strength precision uncertainty will come from using the other lab sections' data).
As for your stress-strain curves...it's probably just ok to include one set of error bars on the figures that represent the uncertainty of the whole curve (it should be reasonably constant over the entire range of strain). You'll have 3 sources of uncertainty as always...accuracy error comes from the noise in the signal that you might have had to remove via filtering, etc. (this you will probably just have to estimate and likely will be small), resolution error from the analog to digital conversion (same as the resolution error of the Instron in lab 1) and you could quantify precision error using the data from your lab section (the two curves won't be exactly the same).