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 $108.44   11.817%   $121.2544   $121.25   $121.25   $121.30  
  Purchase #2 $120.76   12.864%   $136.2946   $136.29   $136.30   $136.30  
  Purchase #3 $144.33   10.485%   $159.463   $159.46   $159.45   $159.50  
  Purchase #4 $103.36   8.337%   $111.9771   $111.98   $112.00   $112.00  
  Purchase #5 $20.73   4.452%   $21.6529   $21.65   $21.65   $21.70  
  Purchase #6 $87.48   7.643%   $94.1661   $94.17   $94.15   $94.20  
  Purchase #7 $129.40   11.836%   $144.7158   $144.72   $144.70   $144.70  
  7 purchase totals = $714.50       $789.5239   $789.52   $789.50   $789.70