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 $243.46   12.810%   $274.6472   $274.65   $274.65   $274.60  
  Purchase #2 $112.94   4.182%   $117.6632   $117.66   $117.65   $117.70  
  Purchase #3 $21.45   7.701%   $23.1019   $23.10   $23.10   $23.10  
  Purchase #4 $186.87   13.603%   $212.2899   $212.29   $212.30   $212.30  
  Purchase #5 $151.79   5.670%   $160.3965   $160.40   $160.40   $160.40  
  Purchase #6 $180.89   4.656%   $189.3122   $189.31   $189.30   $189.30  
  Purchase #7 $38.80   5.332%   $40.8688   $40.87   $40.85   $40.90  
  7 purchase totals = $936.20       $1,018.2797   $1,018.28   $1,018.25   $1,018.30