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 $249.98   4.326%   $260.7941   $260.79   $260.80   $260.80  
  Purchase #2 $65.78   4.833%   $68.9591   $68.96   $68.95   $69.00  
  Purchase #3 $63.44   11.354%   $70.643   $70.64   $70.65   $70.60  
  Purchase #4 $61.32   11.541%   $68.3969   $68.40   $68.40   $68.40  
  Purchase #5 $123.55   11.871%   $138.2166   $138.22   $138.20   $138.20  
  Purchase #6 $108.98   6.315%   $115.8621   $115.86   $115.85   $115.90  
  Purchase #7 $230.98   4.352%   $241.0322   $241.03   $241.05   $241.00  
  7 purchase totals = $904.03       $963.9040   $963.90   $963.90   $963.90