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 $18.85   8.343%   $20.4227   $20.42   $20.40   $20.40  
  Purchase #2 $83.27   8.373%   $90.2422   $90.24   $90.25   $90.20  
  Purchase #3 $187.43   13.788%   $213.2728   $213.27   $213.25   $213.30  
  Purchase #4 $224.48   13.362%   $254.475   $254.48   $254.50   $254.50  
  Purchase #5 $171.68   9.201%   $187.4763   $187.48   $187.50   $187.50  
  Purchase #6 $204.78   5.617%   $216.2825   $216.28   $216.30   $216.30  
  Purchase #7 $151.48   8.144%   $163.8165   $163.82   $163.80   $163.80  
  7 purchase totals = $1,041.97       $1,145.9880   $1,145.99   $1,146.00   $1,146.00