Electric guitars are rather a law unto themselves, as their tonal balance varies so drastically from style to style. However, there are a few general principles to bear in mind. The first is that there will be little other than hum and noise below the guitar's fundamental frequency, so it's often worth filtering below about 80Hz. However, most guitar sounds can be warmed up with a boost at around 125-250Hz, as you can hear from the guitars in Metallica's 'Enter Sandman'.
The other main thing to take into account is that the frequency response of most guitar speaker cabinets rolls off pretty steeply above 4kHz, and so your best choice for emphasising the crispness and attack of guitar sounds is a boost at 3-5kHz -- frequencies to the fore in Nirvana's 'Smells Like Teen Spirit' and Chuck Berry's 'Johnny B Goode', for example. Any boost well above this is likely to increase only noise levels, so if you want an even more cutting lead sound, such as that in Guns & Roses 'Sweet Child Of Mine', you might consider using a psychoacoustic enhancer as well.
Electric Guitar - EQ Fundemental
Cut below 80Hz to reduce unnecessary bassy cabinet boom.
Muddy at 150 to 300 Hz.
Biting at 800Hz to 3kHz.
Fizzy at 5 to 10 kHz.