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 $18.73   7.499%   $20.1346   $20.13   $20.15   $20.10  
  Purchase #2 $39.29   9.811%   $43.1447   $43.14   $43.15   $43.10  
  Purchase #3 $185.31   5.104%   $194.7682   $194.77   $194.75   $194.80  
  Purchase #4 $183.60   11.530%   $204.7691   $204.77   $204.75   $204.80  
  Purchase #5 $84.57   6.826%   $90.3427   $90.34   $90.35   $90.30  
  Purchase #6 $160.35   12.614%   $180.5765   $180.58   $180.60   $180.60  
  Purchase #7 $230.89   3.712%   $239.4606   $239.46   $239.45   $239.50  
  7 purchase totals = $902.74       $973.1964   $973.19   $973.20   $973.20