We encourage you to always up vote content you find that REALLY deserves it, but to answer your question we have the voting set with the following rules. When your VP hits 100% you begin voting since it would be a waste of your maxed out VP to not. If however your VP does not reach 100% in a given 5 hour period, we force your account to vote. Each voting cycle takes ~2% of your VP. VP replenishes at a rate of about 20% per 24 hours. My general rule of thumb is never vote below 90% VP and then let it come back up to 100%, shadowbot is designed that even if you do manually vote, if you give it some breathing time, your VP will replenish to full 100%, which maximizes your curation rewards as well as the influence your votes have. Quality not quantity is what matters on steem.

How it effects votes you receive: Well it does affect your ability to earn Shadow Rank, which directly affects your Pool Priority. Each week Shadow Rank gives points to those who average over 98% VP (meaning they are contributing all or at least most of the votes to other Shadow Casters not voting themselves outside of ShadowBot). It does not have a direct negative effect though, just a lack of a potential positive.