In the first part of the season 3 finale of Buffy: The Vampire Slayer, a flashback shows an interaction between Angel and Drusilla in a church confessional. In the background outside the confessional, there is a clearly displayed cross. One could reasonably assume that there are many other crosses/crucifixes displayed throughout the church.
The Master was not afraid of the sight of a large cross, but was burned when he touched it.
Angel is not visibly affected by the sight of the cross necklace he gives Buffy, but it does burn him.
When Spike is talking to Buffy, he is not visibly affected by the cross around her neck.
So the vast amount of evidence points to the sight of a cross/crucifix not having a negative effect, but physical contact causing burning. But then again:
Angel is shown being warded off by the sight of a cross after he has lost his soul.
Isn't this a question of vampire 'power'. A powerful vampire (the master etc) is strong enough to resist the sight of the cross although the physical affect of burning still happens as it is a physical reaction beyone their control.
Crosses seem more effective at scaring vampires if a person is actively using it, and if the vampire is startled by it. Angel could ignore the cross necklace he gave Buffy, Luke fled from the same cross because he was startled by it, and Angelus was scared off by a cross suddenly waved in his face.
It's never explained canonically, but my understanding is that any contact with crosses causes pain - physical contact causes physical pain, and mental (i.e. sight) contact causes mental pain.
So if the vampire is ignoring the cross or doesn't notice it, they don't get affected (it's not "in contact" mentally). And they can psych themselves up to ignore the pain - just like they are capable of touching the cross physically. It hurts, but they only flinch back if they were not expecting it.