As prcguy has stated, the NEC is very specific about grounding. There is a section that covers "Telecommunications Facilities" and this is where you can find how it should be grounded. I don't recommend going to your electrical panel and tie to the ground there. But a better location is to tie to the ground rod at the electrical meter.
You would also be better off to supply your own ground rods and space them out twice their length. If you use 10 foot long ground rods, space them 20 feet apart. Then your last connection should be to the ground rod at the electrical meter. Use a new clamp to connect to the ground rod at the meter. Do not play with the ground connection going to the meter.
You can also check with your home insurance company. They should also have guide lines about grounding antenna systems. If you don't do it correctly and take a lightning strike that does damage, the insurance company could tell you they won't cover the damage.
So you really need to do your homework and come up with some specifics on what you need to do.
Another good source of grounding information is the R56 specs that Motorola has developed over the years. Most of the cellular companies all seem to follow the same as Motorola has done, with few exceptions.
If you hunt around on the Internet, you should be able to fine a source of the R56 document. It is too large to email. There is way more information there than what your looking for. The document is designed to provide all the information needed to build a radio site. So your going to get a huge document and only need to look at maybe 4 pages or so.
Hope this helps.