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 $38.54   3.772%   $39.9937   $39.99   $40.00   $40.00  
  Purchase #2 $51.33   7.385%   $55.1207   $55.12   $55.10   $55.10  
  Purchase #3 $89.64   8.495%   $97.2549   $97.25   $97.25   $97.30  
  Purchase #4 $139.72   9.713%   $153.291   $153.29   $153.30   $153.30  
  Purchase #5 $246.95   12.369%   $277.4952   $277.50   $277.50   $277.50  
  Purchase #6 $101.87   7.319%   $109.3259   $109.33   $109.35   $109.30  
  Purchase #7 $177.52   11.842%   $198.5419   $198.54   $198.55   $198.50  
  7 purchase totals = $845.57       $931.0233   $931.02   $931.05   $931.00