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 $24.95   9.626%   $27.3517   $27.35   $27.35   $27.40  
  Purchase #2 $64.77   12.267%   $72.7153   $72.72   $72.70   $72.70  
  Purchase #3 $166.84   7.736%   $179.7467   $179.75   $179.75   $179.70  
  Purchase #4 $54.79   7.554%   $58.9288   $58.93   $58.95   $58.90  
  Purchase #5 $111.78   9.214%   $122.0794   $122.08   $122.10   $122.10  
  Purchase #6 $144.56   11.737%   $161.527   $161.53   $161.55   $161.50  
  Purchase #7 $76.96   9.872%   $84.5575   $84.56   $84.55   $84.60  
  7 purchase totals = $644.65       $706.9064   $706.92   $706.95   $706.90