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 $63.96   6.331%   $68.0093   $68.01   $68.00   $68.00  
  Purchase #2 $186.32   13.179%   $210.8751   $210.88   $210.90   $210.90  
  Purchase #3 $26.51   13.488%   $30.0857   $30.09   $30.10   $30.10  
  Purchase #4 $198.91   13.765%   $226.29   $226.29   $226.30   $226.30  
  Purchase #5 $88.29   12.171%   $99.0358   $99.04   $99.05   $99.00  
  Purchase #6 $106.94   13.481%   $121.3566   $121.36   $121.35   $121.40  
  Purchase #7 $162.73   8.252%   $176.1585   $176.16   $176.15   $176.20  
  7 purchase totals = $833.66       $931.8110   $931.83   $931.85   $931.90