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 $234.60   6.712%   $250.3464   $250.35   $250.35   $250.30  
  Purchase #2 $129.49   10.194%   $142.6902   $142.69   $142.70   $142.70  
  Purchase #3 $218.31   4.151%   $227.372   $227.37   $227.35   $227.40  
  Purchase #4 $28.60   10.535%   $31.613   $31.61   $31.60   $31.60  
  Purchase #5 $53.40   12.474%   $60.0611   $60.06   $60.05   $60.10  
  Purchase #6 $165.90   12.742%   $187.039   $187.04   $187.05   $187.00  
  Purchase #7 $138.99   13.107%   $157.2074   $157.21   $157.20   $157.20  
  7 purchase totals = $969.29       $1,056.3291   $1,056.33   $1,056.30   $1,056.30