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 $7.46   6.833%   $7.9697   $7.97   $7.95   $8.00  
  Purchase #2 $36.63   13.224%   $41.474   $41.47   $41.45   $41.50  
  Purchase #3 $204.75   10.557%   $226.3655   $226.37   $226.35   $226.40  
  Purchase #4 $207.39   12.268%   $232.8326   $232.83   $232.85   $232.80  
  Purchase #5 $10.77   10.728%   $11.9254   $11.93   $11.95   $11.90  
  Purchase #6 $45.30   7.386%   $48.6459   $48.65   $48.65   $48.60  
  Purchase #7 $91.55   8.296%   $99.145   $99.15   $99.15   $99.10  
  7 purchase totals = $603.85       $668.3581   $668.37   $668.35   $668.30