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 $169.99   13.561%   $193.0423   $193.04   $193.05   $193.00  
  Purchase #2 $180.67   12.542%   $203.3296   $203.33   $203.35   $203.30  
  Purchase #3 $83.95   8.440%   $91.0354   $91.04   $91.05   $91.00  
  Purchase #4 $47.33   12.467%   $53.2306   $53.23   $53.25   $53.20  
  Purchase #5 $72.83   6.246%   $77.379   $77.38   $77.40   $77.40  
  Purchase #6 $129.33   6.857%   $138.1982   $138.20   $138.20   $138.20  
  Purchase #7 $70.91   5.674%   $74.9334   $74.93   $74.95   $74.90  
  7 purchase totals = $755.01       $831.1485   $831.15   $831.25   $831.00