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 $66.27   13.160%   $74.9911   $74.99   $75.00   $75.00  
  Purchase #2 $19.36   3.307%   $20.0002   $20.00   $20.00   $20.00  
  Purchase #3 $76.32   11.780%   $85.3105   $85.31   $85.30   $85.30  
  Purchase #4 $43.82   5.619%   $46.2822   $46.28   $46.30   $46.30  
  Purchase #5 $164.92   13.530%   $187.2337   $187.23   $187.25   $187.20  
  Purchase #6 $14.93   4.829%   $15.651   $15.65   $15.65   $15.70  
  Purchase #7 $163.31   8.261%   $176.801   $176.80   $176.80   $176.80  
  7 purchase totals = $548.93       $606.2697   $606.26   $606.30   $606.30