Handicapped - to help or not to help?

So I've read before that people in wheelchairs appreciate the kind of niceties you'd extend to people not in wheelchairs (like holding the door open) but that often other offers (such as to push them) are offensive. But twice recently I've witnessed individuals in wheelchairs struggling uphill and it seemed appropriate to offer to help. Both times they've politely declined and I've left feeling uneasy...is there an appropriate way to handle this situation?