I think that either placement would be appropriate, and here's some other placements for you.
If you have several crystals you could place them so that they alternate, with half of them pointing inwards and the other half pointing outwards.
Another placement is to arrange them in a spiral around the outside of the alter with the points pointing slightly inwards towards the center of the alter and leave one large crystal placed standing upright in the middle of the alter. A spiral is a VERY powerful placement and will generate a tremendous amount of energy in towards the alter. The center stone will amplify and reflect back all the energies that are generated by the alter as a whole. This arrangement packs a powerful punch.
And one other arrangement is to place the crystals in a circle around the alter with colored stones placed between each crystal, arranged in the order of the spectrum's color-wheel.