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 $18.75   7.219%   $20.1036   $20.10   $20.10   $20.10  
  Purchase #2 $6.67   4.596%   $6.9766   $6.98   $7.00   $7.00  
  Purchase #3 $138.70   6.670%   $147.9513   $147.95   $147.95   $148.00  
  Purchase #4 $84.93   8.455%   $92.1108   $92.11   $92.10   $92.10  
  Purchase #5 $10.85   5.810%   $11.4804   $11.48   $11.50   $11.50  
  Purchase #6 $233.41   9.385%   $255.3155   $255.32   $255.30   $255.30  
  Purchase #7 $52.28   7.874%   $56.3965   $56.40   $56.40   $56.40  
  7 purchase totals = $545.59       $590.3347   $590.34   $590.35   $590.40