Good question garmenteros, if your levels are low enough when you are practicing alone then hearing protection may not be necessary.
If anyone is curious about your soundlevels, go to radioshak and pick a cheap sound level meter.... I bet you all will be surprised at your loudness levels. These over the counter meters are not exact, but they can give you a great idea of your noise level and exposure without forking over a couple grand.
Keep in mind it is not just how loud the sound is but also factor in how long you are exposed to it. Take this for example, according to Westone (a long time manufacturer of custom ear protection), it takes approximately 1 hour of exposure at 100 dB to cause damage and if you turn it up to 115 dB (which can be quite easy at a live event or practice) it only takes 7.5 mins to cause damage to your hearing. Irreversible damage.
I see it everyday in my office, musicians and non-musicians who cannot hear or have tinnitus (constant ringing). I cannot stress enough, especially you young guys, protect your hearing at gigs, at practice, whenever.
Thanks for reading.