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 $52.97   13.671%   $60.2115   $60.21   $60.20   $60.20  
  Purchase #2 $145.68   10.408%   $160.8424   $160.84   $160.85   $160.80  
  Purchase #3 $224.79   5.396%   $236.9197   $236.92   $236.90   $236.90  
  Purchase #4 $107.72   13.734%   $122.5143   $122.51   $122.50   $122.50  
  Purchase #5 $115.42   5.295%   $121.5315   $121.53   $121.55   $121.50  
  Purchase #6 $189.98   6.825%   $202.9461   $202.95   $202.95   $202.90  
  Purchase #7 $83.83   5.709%   $88.6159   $88.62   $88.60   $88.60  
  7 purchase totals = $920.39       $993.5814   $993.58   $993.55   $993.40