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 $239.91   3.358%   $247.9662   $247.97   $247.95   $248.00  
  Purchase #2 $131.93   12.162%   $147.9753   $147.98   $148.00   $148.00  
  Purchase #3 $134.37   11.497%   $149.8185   $149.82   $149.80   $149.80  
  Purchase #4 $162.51   5.183%   $170.9329   $170.93   $170.95   $170.90  
  Purchase #5 $97.72   8.483%   $106.0096   $106.01   $106.00   $106.00  
  Purchase #6 $147.80   8.213%   $159.9388   $159.94   $159.95   $159.90  
  Purchase #7 $9.91   5.385%   $10.4437   $10.44   $10.45   $10.40  
  7 purchase totals = $924.15       $993.0850   $993.09   $993.10   $993.00