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 $79.70   9.179%   $87.0157   $87.02   $87.00   $87.00  
  Purchase #2 $48.74   9.600%   $53.419   $53.42   $53.40   $53.40  
  Purchase #3 $78.90   6.527%   $84.0498   $84.05   $84.05   $84.00  
  Purchase #4 $142.47   12.551%   $160.3514   $160.35   $160.35   $160.40  
  Purchase #5 $42.70   3.310%   $44.1134   $44.11   $44.10   $44.10  
  Purchase #6 $190.39   7.113%   $203.9324   $203.93   $203.95   $203.90  
  Purchase #7 $15.86   12.514%   $17.8447   $17.84   $17.85   $17.80  
  7 purchase totals = $598.76       $650.7264   $650.72   $650.70   $650.60