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 $47.65   12.561%   $53.6353   $53.64   $53.65   $53.60  
  Purchase #2 $211.47   13.456%   $239.9254   $239.93   $239.95   $239.90  
  Purchase #3 $176.38   10.266%   $194.4872   $194.49   $194.50   $194.50  
  Purchase #4 $5.54   10.304%   $6.1108   $6.11   $6.10   $6.10  
  Purchase #5 $51.30   9.850%   $56.3531   $56.35   $56.35   $56.40  
  Purchase #6 $107.82   10.419%   $119.0538   $119.05   $119.05   $119.10  
  Purchase #7 $135.61   6.855%   $144.9061   $144.91   $144.90   $144.90  
  7 purchase totals = $735.77       $814.4717   $814.48   $814.50   $814.50