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 $244.34   12.654%   $275.2588   $275.26   $275.25   $275.30  
  Purchase #2 $30.73   6.385%   $32.6921   $32.69   $32.70   $32.70  
  Purchase #3 $246.84   12.448%   $277.5666   $277.57   $277.55   $277.60  
  Purchase #4 $67.79   8.525%   $73.5691   $73.57   $73.55   $73.60  
  Purchase #5 $173.69   9.483%   $190.161   $190.16   $190.15   $190.20  
  Purchase #6 $5.93   3.445%   $6.1343   $6.13   $6.15   $6.10  
  Purchase #7 $106.90   12.427%   $120.1845   $120.18   $120.20   $120.20  
  7 purchase totals = $876.22       $975.5664   $975.56   $975.55   $975.70