There are a number of important points raised in your question. This question is very US-centric, as is my answer. Others have already raised the issue of employers who 'self-insure', so this answer does not address that case.
First and foremost, it is illegal for an employer (or potential employer) to ask about your medical situation and it would be highly inappropriate for you to disclose it. This is no different from being asked, or disclosing, your age (or gender preference), and could greatly affect the hiring decision (even though it should not), so do not disclose it.
Secondly, employer-provided 'group' healthcare insurance cannot be denied to anyone in the 'group', so - to use your example - Aetna cannot try to have you removed from the insurance plan, nor can they take into account any pre-existing condition (As an aside, the Affordable Care Act (ACA) attempts to extend this approach to 'individual' insurance).
Thirdly, whether and by how much an employer is affected by an individual employee's specific claims depends on numerous factors but one key factor is the size of the company. A small company (under 100 employees as defined by the ACA) is legally required to be treated differently by an insurance company compared to a larger company. Essentially, there are two ways for an insurance company to set rates; 'community' based or 'experience' based. Small companies must be evaluated using 'community' rating; 'large' companies (over 100) must be evaluated using 'experience' rating.
'Community' based means the rate is set based on the claims history of the 'community', which means, Age, Family Size, Geographic Area, and Tobacco Use. 'Experience' based means the rate is set based on the actual claims history of the actual employee base of the company itself. The reason for this is - once a company attains a certain number of employees, it makes (relative) sense to look at the actual claims history of the company when setting rates. But for a small company, using 'actual' claims history could be so highly skewed by one individual employee's claim history that it is considered unfair practice, and the rates for small companies is therefore required to be set according to the 'community'. Specifically in your case, if you joined a company with (eg) 75 employees, you (your claim activity) would not affect their rates much; if you joined a company with 100,000 employees, you would not affect their rates much either (statistically insignificant); but if you joined a company with 120 employees, you could affect their rates quite substantially.