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 $129.44   6.685%   $138.0931   $138.09   $138.10   $138.10  
  Purchase #2 $70.46   12.354%   $79.1646   $79.16   $79.15   $79.20  
  Purchase #3 $33.55   9.254%   $36.6547   $36.65   $36.65   $36.70  
  Purchase #4 $109.40   13.197%   $123.8375   $123.84   $123.85   $123.80  
  Purchase #5 $133.68   11.256%   $148.727   $148.73   $148.75   $148.70  
  Purchase #6 $127.28   4.425%   $132.9121   $132.91   $132.90   $132.90  
  Purchase #7 $42.83   7.412%   $46.0046   $46.00   $46.00   $46.00  
  7 purchase totals = $646.64       $705.3936   $705.38   $705.40   $705.40