If you do not attend the wedding is it okay to spend less money on a gift than if you were to attend the reception? If so, what percentage less would be acceptable?

No, the gift should be reasonable for your relationship wth the couple.

Gifts should always be because of love.. Free will... what you deem best for both parties... you and couple.. Never feel compelled to give a gift. But also make room to bless a couple when they get married. What Goes around comes around. Give what you can afford. Give cheerfully.. then you will reap reward.. don't let anyone dictate to you what amount you should spend. Just be a blessing.