There are a few ways to interpret "neglecting" iambic pentameter:
* Slight variations: Shakespeare often uses enjambment, where the sense of a line runs over into the next, subtly shifting the rhythm. He also might use inverted word order or extra syllables for emphasis. These are considered minor deviations, and don't actually "break" the iambic pentameter structure.
* Intentional variations: There are a few sonnets where Shakespeare deliberately introduces more significant variations to the rhythm. This might be to create a particular mood, to emphasize certain words, or to simply experiment with the form.
* Unintentional variations: Some scholars argue that Shakespeare might have made occasional minor errors in following the rhythm. However, these are usually very rare and often debated.
It's important to note that Shakespeare was a highly skilled and intentional writer. Even his "deviations" from iambic pentameter were likely deliberate, chosen to create a specific effect.
Therefore, while Shakespeare generally followed the iambic pentameter rhythm, he wasn't afraid to break the mold occasionally for artistic effect. It's hard to say exactly how "often" he neglected it, as the definition of "neglect" is open to interpretation. However, these variations are usually quite subtle and intentional, contributing to the richness and complexity of his sonnets.