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 $246.49   11.770%   $275.5019   $275.50   $275.50   $275.50  
  Purchase #2 $109.43   10.373%   $120.7812   $120.78   $120.80   $120.80  
  Purchase #3 $72.27   7.137%   $77.4279   $77.43   $77.45   $77.40  
  Purchase #4 $214.74   7.225%   $230.255   $230.26   $230.25   $230.30  
  Purchase #5 $165.62   7.352%   $177.7964   $177.80   $177.80   $177.80  
  Purchase #6 $6.70   6.219%   $7.1167   $7.12   $7.10   $7.10  
  Purchase #7 $178.32   10.762%   $197.5108   $197.51   $197.50   $197.50  
  7 purchase totals = $993.57       $1,086.3899   $1,086.40   $1,086.40   $1,086.40