Apart from cutting out sink holes, you should just forget the jigsaw and use a handsaw for all other worktop cuts, learn to cut straight and then finish with a block end plane for a nice finish on wood or to give a good surface to iron on the edging strip. To be honest I generally only use a jig saw where the cut surface can't be seen and I don't care about the blade wander. If you must use a jigsaw the first thing to do is to get the blade guard out of the way so you can see what you are doing. If you take care then with the blade guard out of the way you should still manage not to remove any fingers or larger limbs/extremities.
Fash