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 $28.97   5.331%   $30.5144   $30.51   $30.50   $30.50  
  Purchase #2 $104.53   6.375%   $111.1938   $111.19   $111.20   $111.20  
  Purchase #3 $235.59   11.836%   $263.4744   $263.47   $263.45   $263.50  
  Purchase #4 $76.67   9.487%   $83.9437   $83.94   $83.95   $83.90  
  Purchase #5 $145.88   13.478%   $165.5417   $165.54   $165.55   $165.50  
  Purchase #6 $146.79   5.579%   $154.9794   $154.98   $155.00   $155.00  
  Purchase #7 $77.77   7.690%   $83.7505   $83.75   $83.75   $83.80  
  7 purchase totals = $816.20       $893.3979   $893.38   $893.40   $893.40