I agree with Mike about the proper use of a tap - namely to turn the tap backwards frequently to clear the chips, which helps avoid messing up the tap and also the material into which the threads are being cut. This is especially true for tapping soft metals like Al or Cu. I think an investment in cutting oil is worthwhile, not only for using taps and dies but for drilling, etc. As a DIYer and hobby-ist I have a small bottle that has amounted to a 'lifetime' supply. Aluminum is a bit trickier - there are some cutting fluids specifically for Al, but years ago, an 'old' machinist (probably younger then than I am now) told me his trick was to use hand cream with lots of lanolin in it.
In terms of grinding off a tap to make a bottom tap - I dunno. You can only do that once (and you risk messing up the starting threads). As has been noted, having all three types is useful - one might be able to get away with an intermediate tap plus a bottom tap...
