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 $18.44   8.623%   $20.0301   $20.03   $20.05   $20.00  
  Purchase #2 $124.44   10.201%   $137.1341   $137.13   $137.15   $137.10  
  Purchase #3 $8.93   12.192%   $10.0187   $10.02   $10.00   $10.00  
  Purchase #4 $18.60   7.576%   $20.0091   $20.01   $20.00   $20.00  
  Purchase #5 $153.38   9.347%   $167.7164   $167.72   $167.70   $167.70  
  Purchase #6 $77.52   6.327%   $82.4247   $82.42   $82.40   $82.40  
  Purchase #7 $168.77   8.278%   $182.7408   $182.74   $182.75   $182.70  
  7 purchase totals = $570.08       $620.0739   $620.07   $620.05   $619.90