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 $22.50   7.251%   $24.1315   $24.13   $24.15   $24.10  
  Purchase #2 $133.91   7.731%   $144.2626   $144.26   $144.25   $144.30  
  Purchase #3 $8.70   6.568%   $9.2714   $9.27   $9.25   $9.30  
  Purchase #4 $128.25   9.519%   $140.4581   $140.46   $140.45   $140.50  
  Purchase #5 $48.84   3.203%   $50.4043   $50.40   $50.40   $50.40  
  Purchase #6 $170.32   3.466%   $176.2233   $176.22   $176.20   $176.20  
  Purchase #7 $87.82   7.453%   $94.3652   $94.37   $94.35   $94.40  
  7 purchase totals = $600.34       $639.1164   $639.11   $639.05   $639.20