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 $241.61   13.354%   $273.8746   $273.87   $273.85   $273.90  
  Purchase #2 $164.46   9.118%   $179.4555   $179.46   $179.45   $179.50  
  Purchase #3 $16.83   13.188%   $19.0495   $19.05   $19.05   $19.00  
  Purchase #4 $78.31   7.369%   $84.0807   $84.08   $84.10   $84.10  
  Purchase #5 $176.31   4.464%   $184.1805   $184.18   $184.20   $184.20  
  Purchase #6 $78.28   4.285%   $81.6343   $81.63   $81.65   $81.60  
  Purchase #7 $110.51   7.265%   $118.5386   $118.54   $118.55   $118.50  
  7 purchase totals = $866.31       $940.8137   $940.81   $940.85   $940.80