accounts typically are only disabled for reasons related to the terms of service/rules.
anyways, regarding negative keywords and keyword selection it is often a trial and error process. For starters it is important to setup some tracking, so you know what does generate conversions (how ever you define that) and what keywords typically do not.
When I first setup a keyword campaign, I usually don't have negative keywords included. The reason being, I usually setup a keyword campaign initially with exact phrase match - [using brackets]. If you are using broad match, then I think negative keywords are more important to consider as it is more likely to provide keyword referrals that are not relevant. As you identify those, yes ... then adding negative keywords can be helpful to focus your ppc budget on keywords that have worked in the past.
A couple rule of thumbs to consider.
1) When starting your ppc campaign, start your budget lower at the beginning. As time passes, and you have gathered conversion data ... then consider increasing your budget. The theory here is that you should not "waste" your ppc budget until you have some experience identifying what keywords convert and what keyword don't.
2) The more words within the keyword phrase, the more likely the visitor will be to convert. So if you have one person find your site for [widgets] and another find your site for [blue widgets], the visitor who found your site via [blue widgets] .... more words in the keyword phrase .... is going to be more likely to "convert".
Is it worthwhile to bid on misspelt keyword? How safe is it to bid on such keywords.
Sure. I've never had an issue as long as the keywords/ads are approved by the system. often there is less traffic, but less competition as well. Targeting misspellings can be effective based on my experiences, although just a small percentage.
hth, let us know anything we can answer further to help along!