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 $64.47   3.646%   $66.8206   $66.82   $66.80   $66.80  
  Purchase #2 $173.89   11.840%   $194.4786   $194.48   $194.50   $194.50  
  Purchase #3 $202.53   9.432%   $221.6326   $221.63   $221.65   $221.60  
  Purchase #4 $246.93   10.425%   $272.6725   $272.67   $272.65   $272.70  
  Purchase #5 $98.33   5.787%   $104.0204   $104.02   $104.00   $104.00  
  Purchase #6 $163.80   13.147%   $185.3348   $185.33   $185.35   $185.30  
  Purchase #7 $18.40   10.495%   $20.3311   $20.33   $20.35   $20.30  
  7 purchase totals = $968.35       $1,065.2906   $1,065.28   $1,065.30   $1,065.20