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 $44.98   13.837%   $51.2039   $51.20   $51.20   $51.20  
  Purchase #2 $180.54   10.577%   $199.6357   $199.64   $199.65   $199.60  
  Purchase #3 $66.61   4.312%   $69.4822   $69.48   $69.50   $69.50  
  Purchase #4 $170.81   11.869%   $191.0834   $191.08   $191.10   $191.10  
  Purchase #5 $180.86   11.783%   $202.1707   $202.17   $202.15   $202.20  
  Purchase #6 $11.41   12.276%   $12.8107   $12.81   $12.80   $12.80  
  Purchase #7 $246.28   4.375%   $257.0548   $257.05   $257.05   $257.10  
  7 purchase totals = $901.49       $983.4414   $983.43   $983.45   $983.50