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 $18.31   10.418%   $20.2175   $20.22   $20.20   $20.20  
  Purchase #2 $50.62   7.619%   $54.4767   $54.48   $54.50   $54.50  
  Purchase #3 $9.99   8.493%   $10.8385   $10.84   $10.85   $10.80  
  Purchase #4 $72.80   3.252%   $75.1675   $75.17   $75.15   $75.20  
  Purchase #5 $57.96   8.389%   $62.8223   $62.82   $62.80   $62.80  
  Purchase #6 $159.59   9.640%   $174.9745   $174.97   $174.95   $175.00  
  Purchase #7 $173.46   11.112%   $192.7349   $192.73   $192.75   $192.70  
  7 purchase totals = $542.73       $591.2319   $591.23   $591.20   $591.20