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 $212.49   12.726%   $239.5315   $239.53   $239.55   $239.50  
  Purchase #2 $166.51   6.459%   $177.2649   $177.26   $177.25   $177.30  
  Purchase #3 $44.37   10.611%   $49.0781   $49.08   $49.10   $49.10  
  Purchase #4 $231.54   12.623%   $260.7673   $260.77   $260.75   $260.80  
  Purchase #5 $34.34   13.128%   $38.8482   $38.85   $38.85   $38.80  
  Purchase #6 $231.88   13.841%   $263.9745   $263.97   $263.95   $264.00  
  Purchase #7 $118.60   6.396%   $126.1857   $126.19   $126.20   $126.20  
  7 purchase totals = $1,039.73       $1,155.6502   $1,155.65   $1,155.65   $1,155.70