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 $18.35   13.617%   $20.8487   $20.85   $20.85   $20.80  
  Purchase #2 $207.89   5.106%   $218.5049   $218.50   $218.50   $218.50  
  Purchase #3 $175.46   7.339%   $188.337   $188.34   $188.35   $188.30  
  Purchase #4 $109.42   5.200%   $115.1098   $115.11   $115.10   $115.10  
  Purchase #5 $8.85   3.701%   $9.1775   $9.18   $9.20   $9.20  
  Purchase #6 $115.60   11.437%   $128.8212   $128.82   $128.80   $128.80  
  Purchase #7 $96.62   3.497%   $99.9988   $100.00   $100.00   $100.00  
  7 purchase totals = $732.19       $780.7979   $780.80   $780.80   $780.70