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 $20.71   5.119%   $21.7701   $21.77   $21.75   $21.80  
  Purchase #2 $60.56   6.485%   $64.4873   $64.49   $64.50   $64.50  
  Purchase #3 $23.30   6.109%   $24.7234   $24.72   $24.70   $24.70  
  Purchase #4 $154.49   10.280%   $170.3716   $170.37   $170.35   $170.40  
  Purchase #5 $164.69   9.317%   $180.0342   $180.03   $180.05   $180.00  
  Purchase #6 $228.77   6.495%   $243.6286   $243.63   $243.65   $243.60  
  Purchase #7 $31.55   10.443%   $34.8448   $34.84   $34.85   $34.80  
  7 purchase totals = $684.07       $739.8600   $739.85   $739.85   $739.80