anyways, the mechanic may have meant to say that the instability is caused by blown shocks. When you put lowering springs on stock shocks, they will eventually blow, and cause a bouncy, and rather unstable ride.

IMO, if you are going to get springs, save up for, purchase and install the shocks at the same time unless you don't plan on returning the car to stock in the future, or your stock shocks are already worn/on their way out.

If you are paying someone to install them, then you are going to eventually pay for the labor a 2nd time when you replace the shocks... You could be a man and install them yourself, it isn't that hard with all of the install guides on the 'net, and the with the help of a $20-70 service manual, with correct torque specs so you feel confident in the installation.