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 $57.37   12.154%   $64.3427   $64.34   $64.35   $64.30  
  Purchase #2 $189.92   11.541%   $211.8387   $211.84   $211.85   $211.80  
  Purchase #3 $16.48   5.741%   $17.4261   $17.43   $17.45   $17.40  
  Purchase #4 $226.79   4.619%   $237.2654   $237.27   $237.25   $237.30  
  Purchase #5 $181.56   5.637%   $191.7945   $191.79   $191.80   $191.80  
  Purchase #6 $42.65   6.778%   $45.5408   $45.54   $45.55   $45.50  
  Purchase #7 $236.64   4.297%   $246.8084   $246.81   $246.80   $246.80  
  7 purchase totals = $951.41       $1,015.0166   $1,015.02   $1,015.05   $1,014.90