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 $50.45   12.823%   $56.9192   $56.92   $56.90   $56.90  
  Purchase #2 $141.47   8.415%   $153.3747   $153.37   $153.35   $153.40  
  Purchase #3 $77.32   13.861%   $88.0373   $88.04   $88.05   $88.00  
  Purchase #4 $204.34   6.628%   $217.8837   $217.88   $217.90   $217.90  
  Purchase #5 $128.51   4.849%   $134.7414   $134.74   $134.75   $134.70  
  Purchase #6 $46.85   4.153%   $48.7957   $48.80   $48.80   $48.80  
  Purchase #7 $109.79   10.756%   $121.599   $121.60   $121.60   $121.60  
  7 purchase totals = $758.73       $821.3510   $821.35   $821.35   $821.30