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 $132.62   4.347%   $138.385   $138.39   $138.40   $138.40  
  Purchase #2 $156.43   6.377%   $166.4055   $166.41   $166.40   $166.40  
  Purchase #3 $120.70   12.110%   $135.3168   $135.32   $135.30   $135.30  
  Purchase #4 $124.45   11.299%   $138.5116   $138.51   $138.50   $138.50  
  Purchase #5 $222.36   9.450%   $243.373   $243.37   $243.35   $243.40  
  Purchase #6 $219.97   12.542%   $247.5586   $247.56   $247.55   $247.60  
  Purchase #7 $104.89   3.726%   $108.7982   $108.80   $108.80   $108.80  
  7 purchase totals = $1,081.42       $1,178.3487   $1,178.36   $1,178.30   $1,178.40