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 $236.97   12.804%   $267.3116   $267.31   $267.30   $267.30  
  Purchase #2 $111.29   9.654%   $122.0339   $122.03   $122.05   $122.00  
  Purchase #3 $85.60   8.807%   $93.1388   $93.14   $93.15   $93.10  
  Purchase #4 $116.68   5.685%   $123.3133   $123.31   $123.30   $123.30  
  Purchase #5 $149.91   3.851%   $155.683   $155.68   $155.70   $155.70  
  Purchase #6 $229.65   12.779%   $258.997   $259.00   $259.00   $259.00  
  Purchase #7 $17.54   4.389%   $18.3098   $18.31   $18.30   $18.30  
  7 purchase totals = $947.64       $1,038.7874   $1,038.78   $1,038.80   $1,038.70