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 $181.64   12.117%   $203.6493   $203.65   $203.65   $203.60  
  Purchase #2 $27.73   3.399%   $28.6725   $28.67   $28.65   $28.70  
  Purchase #3 $94.67   5.266%   $99.6553   $99.66   $99.65   $99.70  
  Purchase #4 $48.28   5.773%   $51.0672   $51.07   $51.05   $51.10  
  Purchase #5 $79.78   6.833%   $85.2314   $85.23   $85.25   $85.20  
  Purchase #6 $220.67   8.261%   $238.8995   $238.90   $238.90   $238.90  
  Purchase #7 $250.31   3.454%   $258.9557   $258.96   $258.95   $259.00  
  7 purchase totals = $903.08       $966.1309   $966.14   $966.10   $966.20