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 $6.92   3.231%   $7.1436   $7.14   $7.15   $7.10  
  Purchase #2 $47.52   6.792%   $50.7476   $50.75   $50.75   $50.70  
  Purchase #3 $5.64   10.487%   $6.2315   $6.23   $6.25   $6.20  
  Purchase #4 $155.36   8.687%   $168.8561   $168.86   $168.85   $168.90  
  Purchase #5 $91.53   12.420%   $102.898   $102.90   $102.90   $102.90  
  Purchase #6 $66.33   7.800%   $71.5037   $71.50   $71.50   $71.50  
  Purchase #7 $136.43   6.702%   $145.5735   $145.57   $145.55   $145.60  
  7 purchase totals = $509.73       $552.9540   $552.95   $552.95   $552.90