Pricing a SaaS product?

I developed a SaaS product, DomainProactive, for security monitoring.

I have had it running for a while now with some initial customers using a placeholder price of $5/month.

I am changing this to a tiered price model (at least two tiers) where the higher priced tiers increase the count and frequency of the security tests. I intend to offer a 10% discount for annual subscriptions.

I am interested in your thoughts regarding how many tiers I should have and what they should be priced at or how I would determine a good pricing model. I am also wondering about your thoughts on charging a penny or a dollar less than some ideal price (e.g. $59 instead of $60).