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 $16.50   9.809%   $18.1185   $18.12   $18.10   $18.10  
  Purchase #2 $66.35   12.860%   $74.8826   $74.88   $74.90   $74.90  
  Purchase #3 $37.66   5.547%   $39.749   $39.75   $39.75   $39.70  
  Purchase #4 $199.84   10.315%   $220.4535   $220.45   $220.45   $220.50  
  Purchase #5 $10.40   6.405%   $11.0661   $11.07   $11.05   $11.10  
  Purchase #6 $199.56   9.821%   $219.1588   $219.16   $219.15   $219.20  
  Purchase #7 $174.67   4.104%   $181.8385   $181.84   $181.85   $181.80  
  7 purchase totals = $704.98       $765.2670   $765.27   $765.25   $765.30