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 $49.98   3.309%   $51.6338   $51.63   $51.65   $51.60  
  Purchase #2 $121.51   13.288%   $137.6562   $137.66   $137.65   $137.70  
  Purchase #3 $152.83   10.593%   $169.0193   $169.02   $169.00   $169.00  
  Purchase #4 $126.34   10.180%   $139.2014   $139.20   $139.20   $139.20  
  Purchase #5 $83.95   8.193%   $90.828   $90.83   $90.85   $90.80  
  Purchase #6 $171.92   12.798%   $193.9223   $193.92   $193.90   $193.90  
  Purchase #7 $59.50   10.855%   $65.9587   $65.96   $65.95   $66.00  
  7 purchase totals = $766.03       $848.2197   $848.22   $848.20   $848.20