Short answer, yes if used as any other externalId, though most of the times it's not the best solution.
Planhat has 3 different company IDs:
1) Planhat Internal Id
2) External Id (any id you pick, typically customer id in your own database)
3) SourceId (= SalesforceId)
1 & 2 can be used to send Activities and Metrics to Planhat, but not the source id (3), which is only used internally to sync with Salesforce.
In cases where there is no obvious "customer concept" in your own system (for example several instances may in practice the "same" customer from a business and usage perspective), then you'll need a mapping between instances and some artificial "customer ID", so that usage and users from all these instances will end up on the same customer profile in Planhat.
How and where that is done is of course up to you, but most of our customers opt for keeping that mapping in their own system, as it will help when sending data to Planhat, Salesforce and likely other systems as well. That internal customer ID of yours could be the SalesforceId account ID, but it's highly recommended to use your own IDs or at least treat it as such even if it happens to be the same as Salesforce IDs.