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 $132.81   4.495%   $138.7798   $138.78   $138.80   $138.80  
  Purchase #2 $157.29   13.737%   $178.8969   $178.90   $178.90   $178.90  
  Purchase #3 $106.91   13.441%   $121.2798   $121.28   $121.30   $121.30  
  Purchase #4 $84.45   11.511%   $94.171   $94.17   $94.15   $94.20  
  Purchase #5 $203.62   13.199%   $230.4958   $230.50   $230.50   $230.50  
  Purchase #6 $14.83   7.139%   $15.8887   $15.89   $15.90   $15.90  
  Purchase #7 $189.99   12.299%   $213.3569   $213.36   $213.35   $213.40  
  7 purchase totals = $889.90       $992.8689   $992.88   $992.90   $993.00