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 $104.60   13.306%   $118.5181   $118.52   $118.50   $118.50  
  Purchase #2 $51.93   11.824%   $58.0702   $58.07   $58.05   $58.10  
  Purchase #3 $115.47   9.432%   $126.3611   $126.36   $126.35   $126.40  
  Purchase #4 $235.31   7.405%   $252.7347   $252.73   $252.75   $252.70  
  Purchase #5 $145.96   10.198%   $160.845   $160.85   $160.85   $160.80  
  Purchase #6 $250.28   3.628%   $259.3602   $259.36   $259.35   $259.40  
  Purchase #7 $23.75   4.552%   $24.8311   $24.83   $24.85   $24.80  
  7 purchase totals = $927.30       $1,000.7204   $1,000.72   $1,000.70   $1,000.70