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 $91.39   9.145%   $99.7476   $99.75   $99.75   $99.70  
  Purchase #2 $85.99   5.643%   $90.8424   $90.84   $90.85   $90.80  
  Purchase #3 $83.28   11.662%   $92.9921   $92.99   $93.00   $93.00  
  Purchase #4 $111.95   9.218%   $122.2696   $122.27   $122.25   $122.30  
  Purchase #5 $121.31   9.311%   $132.6052   $132.61   $132.60   $132.60  
  Purchase #6 $240.48   3.682%   $249.3345   $249.33   $249.35   $249.30  
  Purchase #7 $197.89   4.582%   $206.9573   $206.96   $206.95   $207.00  
  7 purchase totals = $932.29       $994.7487   $994.75   $994.75   $994.70