Is steroid injection neccessary for large local reaction to insect sting(20 cm).and is one at greater risk of having systemic reaction next time?

Not particularly: Steroid injections can decrease itching or intensifying symptoms, but a localized arthus reaction that has stopped will eventually fade. Simple antihistamines can help & avoid the side effects of steroids. Allergic reactions tend to replicate themselves the same way per exposure, however, a consult with an allergist to review your history would be prudent.