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 $27.85   4.125%   $28.9988   $29.00   $29.00   $29.00  
  Purchase #2 $39.87   3.450%   $41.2455   $41.25   $41.25   $41.20  
  Purchase #3 $143.60   6.811%   $153.3806   $153.38   $153.40   $153.40  
  Purchase #4 $137.30   4.213%   $143.0844   $143.08   $143.10   $143.10  
  Purchase #5 $13.34   11.361%   $14.8556   $14.86   $14.85   $14.90  
  Purchase #6 $22.45   6.402%   $23.8872   $23.89   $23.90   $23.90  
  Purchase #7 $90.74   3.715%   $94.111   $94.11   $94.10   $94.10  
  7 purchase totals = $475.15       $499.5631   $499.57   $499.60   $499.60