Yes your arm position when taking your blood pressure does affect your blood pressure.
If you have your arm in too high or low of a position then your blood pressure reading can be off by at least 10 percent.
So make sure your arm is resting on a table or other surface and make sure your arm is level with your heart.
When taking your blood pressure to get an accurate reading you should raise your arm to the level of your heart and rest it on a table with the palm of your hand facing upwards.
That's the way my doctor has told me to do the blood pressure reading and my blood pressure reading has always been accurate when doing that.
Also you need to be very still and quiet and do not move your arm or talk or even cough otherwise the blood pressure reading may not be accurate.
You could also place a pillow or small cushion under your arm to help keep your arm comfortable and help your arm get to the right elevation.
It's also best to use the same arm all the time when taking your blood pressure reading because using the other arm could give you inaccurate results.