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 $34.49   12.494%   $38.7992   $38.80   $38.80   $38.80  
  Purchase #2 $231.55   13.131%   $261.9548   $261.95   $261.95   $262.00  
  Purchase #3 $209.60   8.449%   $227.3091   $227.31   $227.30   $227.30  
  Purchase #4 $93.79   4.245%   $97.7714   $97.77   $97.75   $97.80  
  Purchase #5 $45.97   9.168%   $50.1845   $50.18   $50.20   $50.20  
  Purchase #6 $145.33   11.135%   $161.5125   $161.51   $161.50   $161.50  
  Purchase #7 $105.94   13.758%   $120.5152   $120.52   $120.50   $120.50  
  7 purchase totals = $866.67       $958.0467   $958.04   $958.00   $958.10