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 $240.77   6.466%   $256.3382   $256.34   $256.35   $256.30  
  Purchase #2 $51.33   9.815%   $56.368   $56.37   $56.35   $56.40  
  Purchase #3 $125.42   10.415%   $138.4825   $138.48   $138.50   $138.50  
  Purchase #4 $56.62   13.550%   $64.292   $64.29   $64.30   $64.30  
  Purchase #5 $246.56   11.283%   $274.3794   $274.38   $274.40   $274.40  
  Purchase #6 $68.28   5.128%   $71.7814   $71.78   $71.80   $71.80  
  Purchase #7 $216.26   4.367%   $225.7041   $225.70   $225.70   $225.70  
  7 purchase totals = $1,005.24       $1,087.3456   $1,087.34   $1,087.40   $1,087.40