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 $237.56   8.179%   $256.99   $256.99   $257.00   $257.00  
  Purchase #2 $217.40   8.827%   $236.5899   $236.59   $236.60   $236.60  
  Purchase #3 $34.59   9.367%   $37.83   $37.83   $37.85   $37.80  
  Purchase #4 $238.77   12.530%   $268.6879   $268.69   $268.70   $268.70  
  Purchase #5 $78.77   11.232%   $87.6174   $87.62   $87.60   $87.60  
  Purchase #6 $133.90   10.503%   $147.9635   $147.96   $147.95   $148.00  
  Purchase #7 $35.96   13.583%   $40.8444   $40.84   $40.85   $40.80  
  7 purchase totals = $976.95       $1,076.5231   $1,076.52   $1,076.55   $1,076.50