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 $124.28   12.260%   $139.5167   $139.52   $139.50   $139.50  
  Purchase #2 $237.53   10.755%   $263.0764   $263.08   $263.10   $263.10  
  Purchase #3 $208.89   5.502%   $220.3831   $220.38   $220.40   $220.40  
  Purchase #4 $22.97   6.784%   $24.5283   $24.53   $24.55   $24.50  
  Purchase #5 $246.48   9.601%   $270.1445   $270.14   $270.15   $270.10  
  Purchase #6 $145.82   9.523%   $159.7064   $159.71   $159.70   $159.70  
  Purchase #7 $168.95   3.607%   $175.044   $175.04   $175.05   $175.00  
  7 purchase totals = $1,154.92       $1,252.3994   $1,252.40   $1,252.45   $1,252.30