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 $29.93   3.836%   $31.0781   $31.08   $31.10   $31.10  
  Purchase #2 $47.34   11.523%   $52.795   $52.80   $52.80   $52.80  
  Purchase #3 $169.68   8.556%   $184.1978   $184.20   $184.20   $184.20  
  Purchase #4 $6.53   4.108%   $6.7983   $6.80   $6.80   $6.80  
  Purchase #5 $247.36   5.523%   $261.0217   $261.02   $261.00   $261.00  
  Purchase #6 $213.93   5.400%   $225.4822   $225.48   $225.50   $225.50  
  Purchase #7 $189.85   12.576%   $213.7255   $213.73   $213.75   $213.70  
  7 purchase totals = $904.62       $975.0986   $975.11   $975.15   $975.10