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 $55.35   7.108%   $59.2843   $59.28   $59.30   $59.30  
  Purchase #2 $207.57   13.332%   $235.2432   $235.24   $235.25   $235.20  
  Purchase #3 $106.61   11.206%   $118.5567   $118.56   $118.55   $118.60  
  Purchase #4 $196.57   9.869%   $215.9695   $215.97   $215.95   $216.00  
  Purchase #5 $87.30   7.590%   $93.9261   $93.93   $93.95   $93.90  
  Purchase #6 $69.88   4.289%   $72.8772   $72.88   $72.90   $72.90  
  Purchase #7 $22.72   11.373%   $25.3039   $25.30   $25.30   $25.30  
  7 purchase totals = $746.00       $821.1609   $821.16   $821.20   $821.20