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 $12.42   4.126%   $12.9324   $12.93   $12.95   $12.90  
  Purchase #2 $80.32   5.293%   $84.5713   $84.57   $84.55   $84.60  
  Purchase #3 $141.55   13.550%   $160.73   $160.73   $160.75   $160.70  
  Purchase #4 $47.28   11.625%   $52.7763   $52.78   $52.80   $52.80  
  Purchase #5 $125.55   10.228%   $138.3913   $138.39   $138.40   $138.40  
  Purchase #6 $232.45   4.785%   $243.5727   $243.57   $243.55   $243.60  
  Purchase #7 $114.87   10.826%   $127.3058   $127.31   $127.30   $127.30  
  7 purchase totals = $754.44       $820.2798   $820.28   $820.30   $820.30