Rounding to arbitrary leading digits. Not the most elegant solution but does the trick

In[1]:= leadRound[n_, l_ : 2] := 
 Round[n, 10^(Length[IntegerDigits[n]] - l)]

In[2]:= leadRound[56789012]
leadRound[56789012, 2]
leadRound[56789012, 3]
leadRound[56789012, 5]

Out[2]= 57000000

Out[3]= 57000000

Out[4]= 56800000

Out[5]= 56789000

In[6]:= leadRound[56789012, 12]

Out[6]= 56789012

Frank, you are right. The second argument of Round "out of the box" starts from the decimal point. But the OP wants a rounding starting from the leading digit. Which your function does.

The function Round could be used right out of the box. Documentation.

In[1]:= Round[56789012, 10^6]
Out[1]= 57000000

rnd[n_] := Block[{d = 10^(Floor[Log[10, n]] - 1)}, d*Round[n/d]]

rnd[56789012]

57000000