Starting at the start, you must consider the IR35 question before commencing business. You need to ensure you are aware of your situation, if only because if you don’t come under IR35 you wouldn’t want to pay the extra tax.
Assuming you are in business already, the simple but important answer to the question is “not after signing a contract”. Although the Revenue may investigate other elements of your financial relationship their first port of call is your contract. There are therefore very important steps that you can and should take in that contract.
Signing up to a contract then attempting to renegotiate it later is foolhardy and unlikely to ever succeed. Sign in haste, repent at leisure.
Another time to consider IR35 is on contract renewal with your agents.
In black and white – looking at a contract with an eye to IR35 after it has been signed significantly increases chances of tax liability. If your employment arrangement involves limited companies, umbrella companies, managed service companies, personal service companies, agencies and/or composite companies IR35 should be a factor on your mind at all stages of negotiation and contractual wrangling.