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 $19.89   3.661%   $20.6182   $20.62   $20.60   $20.60  
  Purchase #2 $107.62   12.530%   $121.1048   $121.10   $121.10   $121.10  
  Purchase #3 $110.32   9.378%   $120.6658   $120.67   $120.65   $120.70  
  Purchase #4 $151.46   11.736%   $169.2353   $169.24   $169.25   $169.20  
  Purchase #5 $169.67   5.595%   $179.163   $179.16   $179.15   $179.20  
  Purchase #6 $239.45   4.435%   $250.0696   $250.07   $250.05   $250.10  
  Purchase #7 $51.41   3.321%   $53.1173   $53.12   $53.10   $53.10  
  7 purchase totals = $849.82       $913.9740   $913.98   $913.90   $914.00