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 $18.98   5.185%   $19.9641   $19.96   $19.95   $20.00  
  Purchase #2 $39.53   6.434%   $42.0734   $42.07   $42.05   $42.10  
  Purchase #3 $123.64   6.827%   $132.0809   $132.08   $132.10   $132.10  
  Purchase #4 $40.26   11.440%   $44.8657   $44.87   $44.85   $44.90  
  Purchase #5 $220.43   4.206%   $229.7013   $229.70   $229.70   $229.70  
  Purchase #6 $185.35   13.794%   $210.9172   $210.92   $210.90   $210.90  
  Purchase #7 $184.46   3.109%   $190.1949   $190.19   $190.20   $190.20  
  7 purchase totals = $812.65       $869.7975   $869.79   $869.75   $869.90