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 $40.70   5.494%   $42.9361   $42.94   $42.95   $42.90  
  Purchase #2 $12.31   9.608%   $13.4927   $13.49   $13.50   $13.50  
  Purchase #3 $155.25   5.700%   $164.0993   $164.10   $164.10   $164.10  
  Purchase #4 $113.88   11.260%   $126.7029   $126.70   $126.70   $126.70  
  Purchase #5 $94.38   12.459%   $106.1388   $106.14   $106.15   $106.10  
  Purchase #6 $118.26   10.720%   $130.9375   $130.94   $130.95   $130.90  
  Purchase #7 $14.40   5.173%   $15.1449   $15.14   $15.15   $15.10  
  7 purchase totals = $549.18       $599.4522   $599.45   $599.50   $599.30