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 $108.98   13.335%   $123.5125   $123.51   $123.50   $123.50  
  Purchase #2 $186.61   8.209%   $201.9288   $201.93   $201.95   $201.90  
  Purchase #3 $86.44   10.339%   $95.377   $95.38   $95.40   $95.40  
  Purchase #4 $221.44   6.147%   $235.0519   $235.05   $235.05   $235.10  
  Purchase #5 $23.29   4.700%   $24.3846   $24.38   $24.40   $24.40  
  Purchase #6 $32.50   9.568%   $35.6096   $35.61   $35.60   $35.60  
  Purchase #7 $233.93   9.291%   $255.6644   $255.66   $255.65   $255.70  
  7 purchase totals = $893.19       $971.5288   $971.52   $971.55   $971.60