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 $17.39   7.607%   $18.7129   $18.71   $18.70   $18.70  
  Purchase #2 $21.60   12.134%   $24.2209   $24.22   $24.20   $24.20  
  Purchase #3 $85.89   9.573%   $94.1122   $94.11   $94.10   $94.10  
  Purchase #4 $75.97   10.140%   $83.6734   $83.67   $83.65   $83.70  
  Purchase #5 $207.94   7.596%   $223.7351   $223.74   $223.75   $223.70  
  Purchase #6 $24.50   10.313%   $27.0267   $27.03   $27.05   $27.00  
  Purchase #7 $94.34   13.427%   $107.007   $107.01   $107.00   $107.00  
  7 purchase totals = $527.63       $578.4882   $578.49   $578.45   $578.40