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 $140.53   6.588%   $149.7881   $149.79   $149.80   $149.80  
  Purchase #2 $44.40   12.801%   $50.0836   $50.08   $50.10   $50.10  
  Purchase #3 $33.84   12.573%   $38.0947   $38.09   $38.10   $38.10  
  Purchase #4 $18.53   7.747%   $19.9655   $19.97   $19.95   $20.00  
  Purchase #5 $188.30   5.325%   $198.327   $198.33   $198.35   $198.30  
  Purchase #6 $45.55   9.530%   $49.8909   $49.89   $49.90   $49.90  
  Purchase #7 $79.79   4.830%   $83.6439   $83.64   $83.65   $83.60  
  7 purchase totals = $550.94       $589.7937   $589.79   $589.85   $589.80