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 $25.75   10.728%   $28.5125   $28.51   $28.50   $28.50  
  Purchase #2 $190.80   7.518%   $205.1443   $205.14   $205.15   $205.10  
  Purchase #3 $164.82   8.147%   $178.2479   $178.25   $178.25   $178.20  
  Purchase #4 $121.43   8.767%   $132.0758   $132.08   $132.10   $132.10  
  Purchase #5 $143.36   11.603%   $159.9941   $159.99   $160.00   $160.00  
  Purchase #6 $99.38   9.354%   $108.676   $108.68   $108.70   $108.70  
  Purchase #7 $27.54   5.795%   $29.1359   $29.14   $29.15   $29.10  
  7 purchase totals = $773.08       $841.7865   $841.79   $841.85   $841.70