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 $237.57   6.681%   $253.4421   $253.44   $253.45   $253.40  
  Purchase #2 $92.86   12.837%   $104.7804   $104.78   $104.80   $104.80  
  Purchase #3 $58.95   13.267%   $66.7709   $66.77   $66.75   $66.80  
  Purchase #4 $32.44   11.143%   $36.0548   $36.05   $36.05   $36.10  
  Purchase #5 $52.67   6.437%   $56.0604   $56.06   $56.05   $56.10  
  Purchase #6 $69.59   9.329%   $76.0821   $76.08   $76.10   $76.10  
  Purchase #7 $66.59   9.477%   $72.9007   $72.90   $72.90   $72.90  
  7 purchase totals = $610.67       $666.0914   $666.08   $666.10   $666.20