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 $87.96   13.251%   $99.6156   $99.62   $99.60   $99.60  
  Purchase #2 $198.95   5.776%   $210.4414   $210.44   $210.45   $210.40  
  Purchase #3 $165.75   4.162%   $172.6485   $172.65   $172.65   $172.60  
  Purchase #4 $142.91   4.178%   $148.8808   $148.88   $148.90   $148.90  
  Purchase #5 $21.57   8.634%   $23.4324   $23.43   $23.45   $23.40  
  Purchase #6 $245.47   6.387%   $261.1482   $261.15   $261.15   $261.10  
  Purchase #7 $76.31   6.120%   $80.9802   $80.98   $81.00   $81.00  
  7 purchase totals = $938.92       $997.1471   $997.15   $997.20   $997.00