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 $56.43   6.673%   $60.1956   $60.20   $60.20   $60.20  
  Purchase #2 $191.72   11.313%   $213.4093   $213.41   $213.40   $213.40  
  Purchase #3 $27.73   6.531%   $29.541   $29.54   $29.55   $29.50  
  Purchase #4 $83.50   13.753%   $94.9838   $94.98   $95.00   $95.00  
  Purchase #5 $5.86   7.446%   $6.2963   $6.30   $6.30   $6.30  
  Purchase #6 $163.57   10.583%   $180.8806   $180.88   $180.90   $180.90  
  Purchase #7 $23.40   4.592%   $24.4745   $24.47   $24.45   $24.50  
  7 purchase totals = $552.21       $609.7811   $609.78   $609.80   $609.80