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 $19.76   6.231%   $20.9912   $20.99   $21.00   $21.00  
  Purchase #2 $213.66   6.746%   $228.0735   $228.07   $228.05   $228.10  
  Purchase #3 $187.29   5.133%   $196.9036   $196.90   $196.90   $196.90  
  Purchase #4 $195.69   4.845%   $205.1712   $205.17   $205.15   $205.20  
  Purchase #5 $129.84   12.556%   $146.1427   $146.14   $146.15   $146.10  
  Purchase #6 $188.95   10.782%   $209.3226   $209.32   $209.30   $209.30  
  Purchase #7 $207.32   3.546%   $214.6716   $214.67   $214.65   $214.70  
  7 purchase totals = $1,142.51       $1,221.2764   $1,221.26   $1,221.20   $1,221.30