There is ALWAYS a risk of a ban. Most bans (if not all) come from other players reportig you, but you do have to have a few of those to be investigated. Don't bot with something you can't afford to lose.
Also have everything set up well enough can save you many times, ex. QuickIRC plugin, BuddyProwl plugin, set it up correctly and you will be happy enough to kick ass with HB. the more you are prepared the less chance of a report. So meaning make/download a good profile, use usefull plugins, don't bot 1 area for days! etc etc.
one big thing is change it up. dont grind all the way to 85. do some quests,bg's,instances and so on.
Also, dont leave your druid humping an x-mas present for 45min in the middle of Stormvind like I did.
One of the biggest way to get banned other then player reports is using HB for to long at one time. I know a few ppl who will bot a max of 16 hours a day but i dont like to break 12 if i can. I have already lost an account while using HB about 6-8 months ago. I wanted to lvl a pali to play with but didnt want to do it by hand and went for 33 hours straight. Blizz has set up to where they can kinda flag when someone is on for to long to where they can take a look at your account. i thought it was a rumor at first but that was the only way it could have been true for me. I have addons and everything else that would have let me know if someone whispers me ect ect. Just dont be dumb about like i was and you will be fine. since cata has come out i have botted for at least 12 hours a day and been fine so far.
I would think that most ppl just report you without the /w. Why would they want to warn you? I count on something like 3 reports for every /w. I see people trying to f*ck with my bots now and then and they usually say nothing. Love it when you have ppl from the otherside pvp flag themselfs and follow you around hoping for an AOE.