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 $47.27   5.692%   $49.9606   $49.96   $49.95   $50.00  
  Purchase #2 $64.88   4.811%   $68.0014   $68.00   $68.00   $68.00  
  Purchase #3 $195.43   6.187%   $207.5213   $207.52   $207.50   $207.50  
  Purchase #4 $220.62   13.495%   $250.3927   $250.39   $250.40   $250.40  
  Purchase #5 $222.85   4.357%   $232.5596   $232.56   $232.55   $232.60  
  Purchase #6 $131.61   4.237%   $137.1863   $137.19   $137.20   $137.20  
  Purchase #7 $48.51   11.315%   $53.9989   $54.00   $54.00   $54.00  
  7 purchase totals = $931.17       $999.6208   $999.62   $999.60   $999.70