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 $230.46   13.529%   $261.6389   $261.64   $261.65   $261.60  
  Purchase #2 $58.91   12.221%   $66.1094   $66.11   $66.10   $66.10  
  Purchase #3 $69.26   3.466%   $71.6606   $71.66   $71.65   $71.70  
  Purchase #4 $89.96   9.448%   $98.4594   $98.46   $98.45   $98.50  
  Purchase #5 $166.28   8.647%   $180.6582   $180.66   $180.65   $180.70  
  Purchase #6 $106.73   8.234%   $115.5181   $115.52   $115.50   $115.50  
  Purchase #7 $201.29   12.264%   $225.9762   $225.98   $226.00   $226.00  
  7 purchase totals = $922.89       $1,020.0208   $1,020.03   $1,020.00   $1,020.10