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 $17.79   6.208%   $18.8944   $18.89   $18.90   $18.90  
  Purchase #2 $33.85   12.594%   $38.1131   $38.11   $38.10   $38.10  
  Purchase #3 $107.63   11.402%   $119.902   $119.90   $119.90   $119.90  
  Purchase #4 $57.49   8.736%   $62.5123   $62.51   $62.50   $62.50  
  Purchase #5 $243.42   8.427%   $263.933   $263.93   $263.95   $263.90  
  Purchase #6 $106.65   5.525%   $112.5424   $112.54   $112.55   $112.50  
  Purchase #7 $20.78   9.608%   $22.7765   $22.78   $22.80   $22.80  
  7 purchase totals = $587.61       $638.6737   $638.66   $638.70   $638.60