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 $122.29   4.819%   $128.1832   $128.18   $128.20   $128.20  
  Purchase #2 $93.45   7.832%   $100.769   $100.77   $100.75   $100.80  
  Purchase #3 $64.69   8.640%   $70.2792   $70.28   $70.30   $70.30  
  Purchase #4 $36.63   13.715%   $41.6538   $41.65   $41.65   $41.70  
  Purchase #5 $28.97   10.370%   $31.9742   $31.97   $31.95   $32.00  
  Purchase #6 $115.44   5.861%   $122.2059   $122.21   $122.20   $122.20  
  Purchase #7 $94.28   12.348%   $105.9217   $105.92   $105.90   $105.90  
  7 purchase totals = $555.75       $600.9870   $600.98   $600.95   $601.10