If you want seperate reporting for the main domain and it's subdomains, then the seperate GA properties will be fine.
Keep in mind the subdomain traffic will not report as referral traffic to the main domain, and vice versa, together with traffic moving between the subdomains. This is due to the default behaviour of GA regarding the referral exclusion list.
If you want to share the session and originating referrer information of the site visitors between the main domain and it's subdomains if they move between them via site links or similar, then you'll need to use a single GA property on all domains with cookieDomain set to auto (which it should be configured for by default).
It is also usually handy in the above scenario (using a single GA Property), to create another View and apply a filter to prepend the hostname info to the requestURI info in the GA reports so as to split out the pageview information for each domain/subdomain, rather than the default aggregate pageview reporting.