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 $18.65   13.382%   $21.1457   $21.15   $21.15   $21.10  
  Purchase #2 $107.59   4.771%   $112.7231   $112.72   $112.70   $112.70  
  Purchase #3 $57.84   4.366%   $60.3653   $60.37   $60.35   $60.40  
  Purchase #4 $22.41   4.383%   $23.3922   $23.39   $23.40   $23.40  
  Purchase #5 $17.74   8.283%   $19.2094   $19.21   $19.20   $19.20  
  Purchase #6 $135.68   11.137%   $150.7907   $150.79   $150.80   $150.80  
  Purchase #7 $19.28   12.542%   $21.6981   $21.70   $21.70   $21.70  
  7 purchase totals = $379.19       $409.3245   $409.33   $409.30   $409.30