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 $13.50   3.171%   $13.9281   $13.93   $13.95   $13.90  
  Purchase #2 $35.61   11.512%   $39.7094   $39.71   $39.70   $39.70  
  Purchase #3 $7.44   12.265%   $8.3525   $8.35   $8.35   $8.40  
  Purchase #4 $189.45   5.541%   $199.9474   $199.95   $199.95   $199.90  
  Purchase #5 $20.80   5.555%   $21.9554   $21.96   $21.95   $22.00  
  Purchase #6 $150.60   6.442%   $160.3017   $160.30   $160.30   $160.30  
  Purchase #7 $52.99   3.159%   $54.664   $54.66   $54.65   $54.70  
  7 purchase totals = $470.39       $498.8585   $498.86   $498.85   $498.90