I think some folks are confusing the top mounts with the top cups.
The parts which are visible when you open the bonnet (and usually rust quickly unless covers are fitted) are the top cups, not the top mounts. Their only purpose is to stop the front struts from falling down when the car is jacked up (or if you hit a mega pothole at warp speed). With the weight of the car on its wheels, there is normally a small gap between the top cups and the bodyshell; it is higly unlikely that they could be the cause of the squeak and replacing them, whilst cheap and easy unless the threads are rusted solid, won't fix your problem.
The top mounts are located at the top of the suspension strut, between the bodyshell and the strut; they're not visible unless you dismantle the suspension. They can, and do wear - significant wear is an MOT fail, and could be the cause of the squeak, but you'll need to remove the front struts from the car to replace them. Unless they're significanly worn, you can test (and probably fix) this by the spraying a small amount of a
silicone rubber lubricant through the hole beneath the top cup onto the top mount. Do not, under any circumstances, use WD40 for this as it attacks rubber compounds and could cause top mount failure.
A can of silicone spray is very useful when trying to isolate the cause of a squeak; it won't harm most materials so is safe to use, but don't get it anywhere near anything you're planning to paint as even the smallest trace of silicone will spoil a new paint job. For this reason, it's a banned substance in any bodyshop.
You do have to remove the top cups to take out the front struts, and if they're seriously corroded and the threads are damaged, you'll have to replace the front struts, which is a good reason for keeping the nuts which secure the top cups well greased and covered.
Another very common cause of squeaks when turning the steering wheel is friction between the steering wheel trim and the rotating parts of the column, including the wheel itself.
Cars, particularly cheaply built ones, are prone to developing squeaks and rattles as they age; for the most part, these are annoying but benign. Many, many years ago I remeber reading that if you're a person who's obsessed by this, you should restrict yourself to buying only new cars if you can.