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 $234.42   13.455%   $265.9612   $265.96   $265.95   $266.00  
  Purchase #2 $60.64   6.682%   $64.692   $64.69   $64.70   $64.70  
  Purchase #3 $160.74   3.574%   $166.4848   $166.48   $166.50   $166.50  
  Purchase #4 $38.54   7.716%   $41.5137   $41.51   $41.50   $41.50  
  Purchase #5 $68.40   10.642%   $75.6791   $75.68   $75.70   $75.70  
  Purchase #6 $138.69   5.304%   $146.0461   $146.05   $146.05   $146.00  
  Purchase #7 $204.58   13.195%   $231.5743   $231.57   $231.55   $231.60  
  7 purchase totals = $906.01       $991.9512   $991.94   $991.95   $992.00