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 $244.56   11.751%   $273.2982   $273.30   $273.30   $273.30  
  Purchase #2 $106.73   7.634%   $114.8778   $114.88   $114.90   $114.90  
  Purchase #3 $100.69   9.686%   $110.4428   $110.44   $110.45   $110.40  
  Purchase #4 $204.42   4.667%   $213.9603   $213.96   $213.95   $214.00  
  Purchase #5 $35.40   8.560%   $38.4302   $38.43   $38.45   $38.40  
  Purchase #6 $147.53   8.585%   $160.1955   $160.20   $160.20   $160.20  
  Purchase #7 $13.60   10.103%   $14.974   $14.97   $14.95   $15.00  
  7 purchase totals = $852.93       $926.1788   $926.18   $926.20   $926.20