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 $78.77   13.136%   $89.1172   $89.12   $89.10   $89.10  
  Purchase #2 $214.40   6.668%   $228.6962   $228.70   $228.70   $228.70  
  Purchase #3 $13.98   8.219%   $15.129   $15.13   $15.15   $15.10  
  Purchase #4 $199.26   5.304%   $209.8288   $209.83   $209.85   $209.80  
  Purchase #5 $49.48   12.846%   $55.8362   $55.84   $55.85   $55.80  
  Purchase #6 $28.76   13.223%   $32.5629   $32.56   $32.55   $32.60  
  Purchase #7 $13.47   8.776%   $14.6521   $14.65   $14.65   $14.70  
  7 purchase totals = $598.12       $645.8224   $645.83   $645.85   $645.80