Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $246.91   12.285%   $277.2429   $277.24   $277.25   $277.20  
  Purchase #2 $57.98   3.387%   $59.9438   $59.94   $59.95   $59.90  
  Purchase #3 $236.52   9.785%   $259.6635   $259.66   $259.65   $259.70  
  Purchase #4 $231.35   7.806%   $249.4092   $249.41   $249.40   $249.40  
  Purchase #5 $26.25   6.309%   $27.9061   $27.91   $27.90   $27.90  
  Purchase #6 $40.34   10.326%   $44.5055   $44.51   $44.50   $44.50  
  Purchase #7 $238.62   5.852%   $252.584   $252.58   $252.60   $252.60  
  7 purchase totals = $1,077.97       $1,171.2550   $1,171.25   $1,171.25   $1,171.20