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 $70.98   9.873%   $77.9879   $77.99   $78.00   $78.00  
  Purchase #2 $136.32   9.755%   $149.618   $149.62   $149.60   $149.60  
  Purchase #3 $164.26   11.606%   $183.324   $183.32   $183.30   $183.30  
  Purchase #4 $99.73   11.453%   $111.1521   $111.15   $111.15   $111.20  
  Purchase #5 $19.51   4.606%   $20.4086   $20.41   $20.40   $20.40  
  Purchase #6 $87.81   5.548%   $92.6817   $92.68   $92.70   $92.70  
  Purchase #7 $12.38   4.656%   $12.9564   $12.96   $12.95   $13.00  
  7 purchase totals = $590.99       $648.1287   $648.13   $648.10   $648.20