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 $237.35   6.578%   $252.9629   $252.96   $252.95   $253.00  
  Purchase #2 $100.96   8.294%   $109.3336   $109.33   $109.35   $109.30  
  Purchase #3 $185.96   12.126%   $208.5095   $208.51   $208.50   $208.50  
  Purchase #4 $152.25   7.229%   $163.2562   $163.26   $163.25   $163.30  
  Purchase #5 $185.35   12.821%   $209.1137   $209.11   $209.10   $209.10  
  Purchase #6 $36.61   6.399%   $38.9527   $38.95   $38.95   $39.00  
  Purchase #7 $34.42   4.278%   $35.8925   $35.89   $35.90   $35.90  
  7 purchase totals = $932.90       $1,018.0211   $1,018.01   $1,018.00   $1,018.10