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 $237.74   11.266%   $264.5238   $264.52   $264.50   $264.50  
  Purchase #2 $165.98   5.401%   $174.9446   $174.94   $174.95   $174.90  
  Purchase #3 $226.34   9.497%   $247.8355   $247.84   $247.85   $247.80  
  Purchase #4 $88.30   11.541%   $98.4907   $98.49   $98.50   $98.50  
  Purchase #5 $116.63   12.251%   $130.9183   $130.92   $130.90   $130.90  
  Purchase #6 $184.44   9.773%   $202.4653   $202.47   $202.45   $202.50  
  Purchase #7 $151.26   4.424%   $157.9517   $157.95   $157.95   $158.00  
  7 purchase totals = $1,170.69       $1,277.1299   $1,277.13   $1,277.10   $1,277.10