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 $5.54   10.726%   $6.1342   $6.13   $6.15   $6.10  
  Purchase #2 $128.77   10.285%   $142.014   $142.01   $142.00   $142.00  
  Purchase #3 $91.79   13.378%   $104.0697   $104.07   $104.05   $104.10  
  Purchase #4 $70.93   12.399%   $79.7246   $79.72   $79.70   $79.70  
  Purchase #5 $235.75   7.273%   $252.8961   $252.90   $252.90   $252.90  
  Purchase #6 $140.27   4.602%   $146.7252   $146.73   $146.75   $146.70  
  Purchase #7 $149.50   5.649%   $157.9453   $157.95   $157.95   $157.90  
  7 purchase totals = $822.55       $889.5091   $889.51   $889.50   $889.40