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 $177.81   6.812%   $189.9224   $189.92   $189.90   $189.90  
  Purchase #2 $33.48   5.324%   $35.2625   $35.26   $35.25   $35.30  
  Purchase #3 $36.57   9.271%   $39.9604   $39.96   $39.95   $40.00  
  Purchase #4 $178.39   9.118%   $194.6556   $194.66   $194.65   $194.70  
  Purchase #5 $185.34   4.588%   $193.8434   $193.84   $193.85   $193.80  
  Purchase #6 $120.63   7.456%   $129.6242   $129.62   $129.60   $129.60  
  Purchase #7 $5.67   13.290%   $6.4235   $6.42   $6.40   $6.40  
  7 purchase totals = $737.89       $789.6920   $789.68   $789.60   $789.70