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 $49.86   12.228%   $55.9569   $55.96   $55.95   $56.00  
  Purchase #2 $115.87   5.362%   $122.0829   $122.08   $122.10   $122.10  
  Purchase #3 $103.99   12.873%   $117.3766   $117.38   $117.40   $117.40  
  Purchase #4 $72.45   12.136%   $81.2425   $81.24   $81.25   $81.20  
  Purchase #5 $31.29   6.744%   $33.4002   $33.40   $33.40   $33.40  
  Purchase #6 $108.35   12.329%   $121.7085   $121.71   $121.70   $121.70  
  Purchase #7 $24.33   12.359%   $27.3369   $27.34   $27.35   $27.30  
  7 purchase totals = $506.14       $559.1045   $559.11   $559.15   $559.10