It is prohibited from the Torah to squeeze grapes and olives for their juice. The Hachamim extended this prohibition to include any fruit or vegetable commonly squeezed for its juice. Today, this would apply to a very wide range of fruits and vegetables.
The Shulhan Aruch in Siman 320 brings the Halacha that permits squeezing any fruit, including grapes and olives, if the juice is extracted directly over e a solid food. For example, one may squeeze a lemon over a piece of fish or a salad, (even according to the opinion that the prohibition of squeezing applies to lemons, which is not the accepted ruling for Sepharadim, who hold there is no problem to squeeze lemon juice directly into an empty cup.) Similarly, it is permitted to press a piece of bread onto an oily piece of fish to extract its liquid directly into the solid food.
However, Maran proceeds to bring the opinion of Rabbenu Hannanael who is strict and does not permit squeezing juice onto solid foods, and one who does so is liable to bring a Korban Hatat. Maran did not bring this strict opinion in the "Stam" (unattributed opinion), only as a "Yesh Omrim." Therefore, clearly Maran is lenient.
Nevertheless, Hacham Bension writes that since Rabbenu Hannanael is one of the foremost earliest Rishonim, his opinion cannot be discounted altogether. Therefore, he rules that "Hamachmir Tavo Alav Beracha"-one who is strict and refrains from squeezing fruit over solid foods is worthy of blessing. This is the opinion of the Mishna Berura, as well.
SUMMARY
Although it is permitted to squeeze fruit over solid foods, one who refrains from doing so is praiseworthy.
