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 $247.77   7.373%   $266.0381   $266.04   $266.05   $266.00  
  Purchase #2 $83.73   5.537%   $88.3661   $88.37   $88.35   $88.40  
  Purchase #3 $8.56   13.479%   $9.7138   $9.71   $9.70   $9.70  
  Purchase #4 $230.36   5.562%   $243.1726   $243.17   $243.15   $243.20  
  Purchase #5 $101.27   6.375%   $107.726   $107.73   $107.75   $107.70  
  Purchase #6 $186.75   12.521%   $210.133   $210.13   $210.15   $210.10  
  Purchase #7 $101.73   3.481%   $105.2712   $105.27   $105.25   $105.30  
  7 purchase totals = $960.17       $1,030.4208   $1,030.42   $1,030.40   $1,030.40