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 $246.95   3.149%   $254.7265   $254.73   $254.75   $254.70  
  Purchase #2 $170.63   10.731%   $188.9403   $188.94   $188.95   $188.90  
  Purchase #3 $231.27   8.218%   $250.2758   $250.28   $250.30   $250.30  
  Purchase #4 $234.68   12.787%   $264.6885   $264.69   $264.70   $264.70  
  Purchase #5 $26.35   12.703%   $29.6972   $29.70   $29.70   $29.70  
  Purchase #6 $46.96   10.755%   $52.0105   $52.01   $52.00   $52.00  
  Purchase #7 $63.35   13.718%   $72.0404   $72.04   $72.05   $72.00  
  7 purchase totals = $1,020.19       $1,112.3792   $1,112.39   $1,112.45   $1,112.30