Short answer is Yes, if used as any other externalId, though most of the times - it's not the best solution.
Planhat has three different company id's:
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 have 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 if of course up to you, but most of our customers prefer keeping that mapping in their own system, as it will help when sending data to other systems (Planhat, Salesforce or any other systems).
Your internal customer id, could be your SalesforceId account id, but it's highly recommended to use your own id's.