To complicate matters, there are two main types of toms; single-headed and double-headed. Single-headed toms (also called concert toms) are far less popular today than they were in the '70s. We can thank the demise of disco and corporate pop-rock for that . Although, small single-headed toms are still popular. Double-headed toms have a fuller, more complex sound, where single-headed toms are thin, without much body.
There are three schools of thought on tom tuning: 1) Tune the bottom head the same as the top, 2) tune the bottom head higher, and 3) tune the top head higher. Each of these results in a different overall sound. Some believe that the top head is tuned so it feels good to play on, and the bottom head is tuned to generate the main pitch of the drum. [Personally, I don't subscribe to that. Both heads equally affect pitch and feel.]
When both heads are tuned to the same pitch, the drum can resonate to its fullest because the heads are not fighting each other for control of the pitch. When one head is tuned differently than the other, it can produce a rise or drop in the overall pitch as it decays. Both are equally popular (my guess).
Now, when you go to tune each head, it can be hard to tell the pitch of each. The best way to do this is to mute the head you're not tuning. Set the drum on a cushion or on the floor while you tune, then only tap the top head to hear its pitch. This pitch will be much different from the final sound of the drum when it's mounted. To match the pitch of each head, lay the drum on its side. Hold your hand against one head while softly tapping the other. Then reverse that and note any differences in pitch.
One trick that some drummers use to create a sound that drops
in pitch as it decays is to detune one or two adjacent lugs slightly. Note
that this reduces sustain a bit.