I think a better strategy is to simply de-emphasise the optional fields. This can be done by showing the label in grey rather than black text. Putting "optional

I think a better strategy is to simply de-emphasise the optional fields. This can be done by showing the label in grey rather than black text. Putting "optional" in the input box itself won't work, because frequently all your input boxes will have "hint" text in there already. That said, red asterisks are fairly common and I think users understand them - so there are good reasons for maintaining the status quo.