If you change the scale of a model (many or few with a reasonable number of vertices) every frame this does indeed have a notable impact on the framerate (the value is not really important, only bigger models result in more pixels to be drawn), if you only do it once or a few times or only for a few models it's perfectly fine.
Generally speaking just go ahead and do it and only optimize if necessary (one optimization - what I normally do - is to scale the model in the vertex shader via vecSkill41).