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 $83.30   7.736%   $89.7441   $89.74   $89.75   $89.70  
  Purchase #2 $19.26   9.497%   $21.0891   $21.09   $21.10   $21.10  
  Purchase #3 $6.67   9.184%   $7.2826   $7.28   $7.30   $7.30  
  Purchase #4 $16.91   11.819%   $18.9086   $18.91   $18.90   $18.90  
  Purchase #5 $214.48   12.619%   $241.5452   $241.55   $241.55   $241.50  
  Purchase #6 $196.64   6.872%   $210.1531   $210.15   $210.15   $210.20  
  Purchase #7 $219.63   12.171%   $246.3612   $246.36   $246.35   $246.40  
  7 purchase totals = $756.89       $835.0839   $835.08   $835.10   $835.10