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 $21.90   7.168%   $23.4698   $23.47   $23.45   $23.50  
  Purchase #2 $46.33   7.753%   $49.922   $49.92   $49.90   $49.90  
  Purchase #3 $135.91   12.797%   $153.3024   $153.30   $153.30   $153.30  
  Purchase #4 $120.36   7.234%   $129.0668   $129.07   $129.05   $129.10  
  Purchase #5 $147.61   7.223%   $158.2719   $158.27   $158.25   $158.30  
  Purchase #6 $219.50   5.327%   $231.1928   $231.19   $231.20   $231.20  
  Purchase #7 $75.92   12.788%   $85.6286   $85.63   $85.65   $85.60  
  7 purchase totals = $767.53       $830.8543   $830.85   $830.80   $830.90