Would I be able to use a .75 amp Battery tender to store my 06 Sportster over the winter in a heated garage
You sure would, the smaller one like you asked about (.75) is the best one as long as it has a float charge (maintenance feature) so it doesn't overcharge the battery. I generally only leave it on until the light turns green and shut it off then. Do this about once a week and all will be good. As far as technology goes, I still don't trust that float feature on the chargers but that's just me. If you have an electric garage door, you can hook the charger up to the light circuit on the door and it will give the battery a charge for 4 minutes every time the door is operated but otherwise be turned off..