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 $24.78   12.556%   $27.8914   $27.89   $27.90   $27.90  
  Purchase #2 $44.81   12.725%   $50.5121   $50.51   $50.50   $50.50  
  Purchase #3 $106.57   9.828%   $117.0437   $117.04   $117.05   $117.00  
  Purchase #4 $156.72   7.407%   $168.3283   $168.33   $168.35   $168.30  
  Purchase #5 $147.75   7.625%   $159.0159   $159.02   $159.00   $159.00  
  Purchase #6 $183.59   7.819%   $197.9449   $197.94   $197.95   $197.90  
  Purchase #7 $167.32   12.762%   $188.6734   $188.67   $188.65   $188.70  
  7 purchase totals = $831.54       $909.4097   $909.40   $909.40   $909.30