Controlled Traces Length in SD card
The speed of a signal in a PCB is about half the speed of light: 15cm/ns .

The maximum difference between two signals, or more precisely, between the clock signal and any other data signal depends on the timing specification.  At all times the hold and setup times vs. the clock must be respected.

At 50MHz, a clock cycle is 20ns, which corresponds to a length of about 300cm.
At 50MHz, the setup time is 6ns and the hold time is 2ns relative to the clock signal (according to a document about SDCards).

Changing the data output at the same time as the clock signal would make it difficult to respect the hold time without introducing a delay of 2ns on the data signals.  Delaying the data signals with a 30cm trace length each will take up some space on your board.
Changing the data output on the inactive edge of the clock leaves a margin of about 8ns (10ns - 2ns), which corresponds to 120cm.  I would not expect to have that much difference in trace lengths.

So in practice it is unlikely that you have to care about the trace lengths for your SD Card.

For your information: a very long time ago a colleague of mine needed a delay of "exactly" 1ns.  Following my proposal, that delay was added by drawing a 15cm line in a 1cm x 1cm square on the board.  This was the first thing that my colleague measured when the board came back: he observed a delay of "exactly" 1ns.  Which confirmed that the speed of the signal in the board is 15cm/ns.  That can vary a bit of course depending on the electrical properties of the board, but it will be very close to that number.