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 $129.39   6.451%   $137.7369   $137.74   $137.75   $137.70  
  Purchase #2 $8.79   5.653%   $9.2869   $9.29   $9.30   $9.30  
  Purchase #3 $80.73   8.498%   $87.5904   $87.59   $87.60   $87.60  
  Purchase #4 $205.91   6.491%   $219.2756   $219.28   $219.30   $219.30  
  Purchase #5 $214.60   6.582%   $228.725   $228.73   $228.75   $228.70  
  Purchase #6 $98.80   11.340%   $110.0039   $110.00   $110.00   $110.00  
  Purchase #7 $218.99   10.144%   $241.2043   $241.20   $241.20   $241.20  
  7 purchase totals = $957.21       $1,033.8230   $1,033.83   $1,033.90   $1,033.80