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 $245.33   6.359%   $260.9305   $260.93   $260.95   $260.90  
  Purchase #2 $201.42   8.389%   $218.3171   $218.32   $218.30   $218.30  
  Purchase #3 $16.30   10.317%   $17.9817   $17.98   $18.00   $18.00  
  Purchase #4 $81.47   6.633%   $86.8739   $86.87   $86.85   $86.90  
  Purchase #5 $88.82   9.114%   $96.9151   $96.92   $96.90   $96.90  
  Purchase #6 $43.58   5.550%   $45.9987   $46.00   $46.00   $46.00  
  Purchase #7 $226.68   10.834%   $251.2385   $251.24   $251.25   $251.20  
  7 purchase totals = $903.60       $978.2555   $978.26   $978.25   $978.20