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 $24.63   11.720%   $27.5166   $27.52   $27.50   $27.50  
  Purchase #2 $213.56   8.789%   $232.3298   $232.33   $232.35   $232.30  
  Purchase #3 $208.32   9.408%   $227.9187   $227.92   $227.90   $227.90  
  Purchase #4 $189.43   13.139%   $214.3192   $214.32   $214.30   $214.30  
  Purchase #5 $186.54   12.534%   $209.9209   $209.92   $209.90   $209.90  
  Purchase #6 $229.33   5.588%   $242.145   $242.15   $242.15   $242.10  
  Purchase #7 $110.86   4.424%   $115.7644   $115.76   $115.75   $115.80  
  7 purchase totals = $1,162.67       $1,269.9146   $1,269.92   $1,269.85   $1,269.80