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 $18.89   6.528%   $20.1231   $20.12   $20.10   $20.10  
  Purchase #2 $36.47   8.384%   $39.5276   $39.53   $39.55   $39.50  
  Purchase #3 $143.99   6.363%   $153.1521   $153.15   $153.15   $153.20  
  Purchase #4 $49.46   7.309%   $53.075   $53.08   $53.10   $53.10  
  Purchase #5 $6.56   4.646%   $6.8648   $6.86   $6.85   $6.90  
  Purchase #6 $172.59   7.212%   $185.0372   $185.04   $185.05   $185.00  
  Purchase #7 $166.95   6.376%   $177.5947   $177.59   $177.60   $177.60  
  7 purchase totals = $594.91       $635.3745   $635.37   $635.40   $635.40