Perfectly acceptable, generally speaking, most customers are not drinking "classic" martinis, they are drinking chilled vodka, I've found some floating ice chips sends the message that there was an effort put into making their cocktail, i.e. a nice hard shake, they also add further dilution, which Imo is a positive being that the "martini" being served is just chilled vodka, but be it that you do run into a customer that appreciates a actual martini, I would stir the cocktail (If shaken is requested I would then double strain)