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 $138.66   7.519%   $149.0858   $149.09   $149.10   $149.10  
  Purchase #2 $5.86   9.847%   $6.437   $6.44   $6.45   $6.40  
  Purchase #3 $144.26   12.345%   $162.0689   $162.07   $162.05   $162.10  
  Purchase #4 $162.81   10.148%   $179.332   $179.33   $179.35   $179.30  
  Purchase #5 $102.70   9.583%   $112.5417   $112.54   $112.55   $112.50  
  Purchase #6 $174.26   11.649%   $194.5595   $194.56   $194.55   $194.60  
  Purchase #7 $87.25   6.503%   $92.9239   $92.92   $92.90   $92.90  
  7 purchase totals = $815.80       $896.9488   $896.95   $896.95   $896.90