The issue with regard repeat prescriptions of oral steroids beyond age five for asthma is that it is generally the case that it indicates a recurrent life-threatening condition and as such, a bar to entry.
That said, if the oral steroids were prescribed due to an upper respiratory tract infection rather than asthma, then that is not an issue.
The tip is to simply be honest because it will otherwise bite you in the arse sooner or later and if it's as you state, then you'll be passed fit anyway.
Fail to declare, prepare to be discharged permanently for making a fraudulent application at any point thereafter. When you fill out the medical questionnaire you also sign a consent form granting the service full access to your entire civilian medical records. Beware paying heed to plantpots that believe it's a good idea to tell porkies when embarking on a potential lifelong career.