Yes you need to otherwise you will have to create new ones manually..

PS Cmdlet to export the existing transport rules in On-Premise:

$onpremrule = Export-TransportRuleCollection

Set-Content -Path “C:\Rules.xml” -Value $onpremrule.FileData -Encoding Byte

Import will overwrite the existing transport rules in O365, please take a backup of O365 rules before importing On-premises rules to O365.

PS Cmdlet to import the exported transport rules in Office 365:

[Byte[]$O365 = Get-Content -Path “C:\Rules.xml” -Encoding Byte -ReadCount 0

Import-TransportRuleCollection -FileData $O365

Please refer below article for more details.

Reference Article: http://messagingpro.in/export-transport-rules-from-on-premise-exchange-2010-or-later-and-import-it-to-office-365/

Does this need to be done even if Centralised Transport is configured? Centralised Transport is where all emails from O365 are routed to the on-prem Exchange.

Transport rules is specific to server whether its onpremise or O365.

So if Centralised Transport is enabled then and all mailboxes are on Onpremise then no need to export-import rules to O365.

All processing will be from onpremise since mx is pointed to onpremise and centralized mail flow will trigger from onpremise only.

Note: In case all mailboxes are moved to O365 then export-import is required.

But we will have a mix of users in on-premise and online. 90% users on-premise and 10% in EOL.

Need transport rules and disclaimers on both sides?

If the rule is globally set the you need them on both sides.

If the rule is specific to a mailbox, then depends where the mailbox resides.