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 $18.92   10.319%   $20.8724   $20.87   $20.85   $20.90  
  Purchase #2 $75.62   11.789%   $84.5348   $84.53   $84.55   $84.50  
  Purchase #3 $191.62   4.529%   $200.2985   $200.30   $200.30   $200.30  
  Purchase #4 $90.56   10.377%   $99.9574   $99.96   $99.95   $100.00  
  Purchase #5 $7.40   5.658%   $7.8187   $7.82   $7.80   $7.80  
  Purchase #6 $19.84   3.592%   $20.5527   $20.55   $20.55   $20.60  
  Purchase #7 $94.53   3.328%   $97.676   $97.68   $97.70   $97.70  
  7 purchase totals = $498.49       $531.7105   $531.71   $531.70   $531.80