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 $19.74   8.417%   $21.4015   $21.40   $21.40   $21.40  
  Purchase #2 $5.26   10.323%   $5.803   $5.80   $5.80   $5.80  
  Purchase #3 $234.73   9.128%   $256.1562   $256.16   $256.15   $256.20  
  Purchase #4 $127.26   7.381%   $136.6531   $136.65   $136.65   $136.70  
  Purchase #5 $193.52   9.805%   $212.4946   $212.49   $212.50   $212.50  
  Purchase #6 $24.52   4.827%   $25.7036   $25.70   $25.70   $25.70  
  Purchase #7 $244.29   12.149%   $273.9688   $273.97   $273.95   $274.00  
  7 purchase totals = $849.32       $932.1808   $932.17   $932.15   $932.30