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 $246.73   11.258%   $274.5069   $274.51   $274.50   $274.50  
  Purchase #2 $53.87   10.123%   $59.3233   $59.32   $59.30   $59.30  
  Purchase #3 $18.50   13.374%   $20.9742   $20.97   $20.95   $21.00  
  Purchase #4 $51.87   12.534%   $58.3714   $58.37   $58.35   $58.40  
  Purchase #5 $64.41   7.676%   $69.3541   $69.35   $69.35   $69.40  
  Purchase #6 $104.40   6.290%   $110.9668   $110.97   $110.95   $111.00  
  Purchase #7 $238.93   13.771%   $271.8331   $271.83   $271.85   $271.80  
  7 purchase totals = $778.71       $865.3298   $865.32   $865.25   $865.40