IMHO, I found that if the tension/stitches are ok when stitching a straight line, the tension is ok.
If you start having problems when you do curvy lines it tends to be a speed of machine vs speed of operator problem. Either the speed of the machine is not fast enough for how fast you are moving the fabric, or you have to slow down the movement of the fabric to catch up to the speed of your machine.
This sometimes shows up when you start getting more confident and start moving faster without speeding up the stitch speed.
Keep practising. It gets easier.