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 $235.43   6.393%   $250.481   $250.48   $250.50   $250.50  
  Purchase #2 $220.99   11.751%   $246.9585   $246.96   $246.95   $247.00  
  Purchase #3 $238.32   5.649%   $251.7827   $251.78   $251.80   $251.80  
  Purchase #4 $32.70   3.749%   $33.9259   $33.93   $33.95   $33.90  
  Purchase #5 $180.92   9.233%   $197.6243   $197.62   $197.60   $197.60  
  Purchase #6 $118.26   5.296%   $124.523   $124.52   $124.50   $124.50  
  Purchase #7 $180.38   9.475%   $197.471   $197.47   $197.45   $197.50  
  7 purchase totals = $1,207.00       $1,302.7664   $1,302.76   $1,302.75   $1,302.80