Mickey2

No you don't. What type of machine do you have? Janey sums it up too; It has mostly to do with getting tension right and matching needle and thread size to a suitable combination as well as to the fabric. Sharp point needles give the straightest stitching, ball point needles tend to slide between the threads in the weave (They are made to behave this way). Some machines don't handle top stitch weights well, but most of the vintage models do fine.

Another factor can be cleaning and oiling, on an old machine you sometimes have to do an extra thorough cleaning in the bobbin and feed dog area. A smooth running mechanism will help the feed dogs movement and stitch lenght setting. The easiest thread to get right is Gütermann sew-all polyester, it's ideal for test sewing and tension setting as well. It's strong and works on most fabrics. Other threads I like are good quality mercerized cotton. I use top stitch and extra strenght threads too, and I usually have to adjust bobbin tension going from one thread weight to another.

I think everything has been mentioned but to sum up the general:

1. Matching thread weight and needle size to each other and to the fabric you are working with.

2. Regular maintanance; clean bobbin area; take out the bobbin case, race and hook. Clean the feed dogs under the needle plate. Oil the machine, everything runs smoother after a cleaning and oiling. Scrape corners and grooves clean with a tooth pick or cocktail pin.
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3. If you are very picky you need to do a bit of testing; correct tension and needle size will get the two threads to interlock perfectly in the middle of the fabric layers. It will make each stitch form in a straight line. It's a bit trickier with light weight fabrics and few layers, but it's still mostly about getting the right needle and thread for the job.

4. Sewing a straight seam is usually easy, but some fabrics can be a challenge (slippery velvet, stretchy jersey blends,...). The trick is to always iron fabric before you start working with it, steam press seam folds, and keep your machine clean and well oiled. Using a fabric guide can help keep a straight line, but for many jobs your eye is enough. Do a bit of test sewing and you will get to know your macine. If you are new to sewing all you need to do is practise a bit.

I think it's all mentioned earlier in the thread, but keep these things in mind when you run into trouble. It get's repetitive, but go through the basic steps and you will get fine results.