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 $28.96   6.654%   $30.887   $30.89   $30.90   $30.90  
  Purchase #2 $103.46   4.369%   $107.9802   $107.98   $108.00   $108.00  
  Purchase #3 $181.71   3.324%   $187.75   $187.75   $187.75   $187.80  
  Purchase #4 $34.58   5.150%   $36.3609   $36.36   $36.35   $36.40  
  Purchase #5 $32.97   7.706%   $35.5107   $35.51   $35.50   $35.50  
  Purchase #6 $30.49   5.335%   $32.1166   $32.12   $32.10   $32.10  
  Purchase #7 $95.94   4.785%   $100.5307   $100.53   $100.55   $100.50  
  7 purchase totals = $508.11       $531.1361   $531.14   $531.15   $531.20