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 $245.68   12.196%   $275.6431   $275.64   $275.65   $275.60  
  Purchase #2 $18.61   5.562%   $19.6451   $19.65   $19.65   $19.60  
  Purchase #3 $114.94   9.735%   $126.1294   $126.13   $126.15   $126.10  
  Purchase #4 $238.37   11.711%   $266.2855   $266.29   $266.30   $266.30  
  Purchase #5 $195.47   7.771%   $210.66   $210.66   $210.65   $210.70  
  Purchase #6 $109.79   5.525%   $115.8559   $115.86   $115.85   $115.90  
  Purchase #7 $62.28   8.361%   $67.4872   $67.49   $67.50   $67.50  
  7 purchase totals = $985.14       $1,081.7062   $1,081.72   $1,081.75   $1,081.70