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 $6.70   10.478%   $7.402   $7.40   $7.40   $7.40  
  Purchase #2 $191.78   12.768%   $216.2665   $216.27   $216.25   $216.30  
  Purchase #3 $127.89   6.543%   $136.2578   $136.26   $136.25   $136.30  
  Purchase #4 $189.95   12.331%   $213.3727   $213.37   $213.35   $213.40  
  Purchase #5 $124.60   7.774%   $134.2864   $134.29   $134.30   $134.30  
  Purchase #6 $31.91   6.842%   $34.0933   $34.09   $34.10   $34.10  
  Purchase #7 $85.76   3.250%   $88.5472   $88.55   $88.55   $88.50  
  7 purchase totals = $758.59       $830.2259   $830.23   $830.20   $830.30