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 $227.93   12.323%   $256.0178   $256.02   $256.00   $256.00  
  Purchase #2 $218.74   6.311%   $232.5447   $232.54   $232.55   $232.50  
  Purchase #3 $107.52   12.738%   $121.2159   $121.22   $121.20   $121.20  
  Purchase #4 $239.45   12.652%   $269.7452   $269.75   $269.75   $269.70  
  Purchase #5 $167.29   5.758%   $176.9226   $176.92   $176.90   $176.90  
  Purchase #6 $246.66   13.682%   $280.408   $280.41   $280.40   $280.40  
  Purchase #7 $144.40   9.323%   $157.8624   $157.86   $157.85   $157.90  
  7 purchase totals = $1,351.99       $1,494.7166   $1,494.72   $1,494.65   $1,494.60