I think I would model them as dividends because you don't know whether buybacks can be done at prices that are accretive or dilutive in the future. The most important criteria is what you think of managements capital allocation skills, if you think they are really smart and good at it, then buybacks are opportunistic upside. If you don't trust management to buy back stock at reasonable prices, then it can be a negative since some happily overpay to drive the price up so they can unload their options for bigger profits.
With Apple I think Tim Cook has done a great job but historically it's stock has been clearly undervalued so he couldn't go wrong. Today at $197 I think it's either overvalued or fairly valued, and last quarter Apple was still buying back shares last quarter within the $135 to $155 range. If this quarter he's buying back shares above that range I'd say he might not get it. He may have just heard "buy back stock" from Buffett not the "when it's undervalued" part.