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 $138.50   7.398%   $148.7462   $148.75   $148.75   $148.70  
  Purchase #2 $70.81   13.418%   $80.3113   $80.31   $80.30   $80.30  
  Purchase #3 $159.77   4.117%   $166.3477   $166.35   $166.35   $166.30  
  Purchase #4 $77.66   9.338%   $84.9119   $84.91   $84.90   $84.90  
  Purchase #5 $112.56   5.301%   $118.5268   $118.53   $118.55   $118.50  
  Purchase #6 $20.34   7.513%   $21.8681   $21.87   $21.85   $21.90  
  Purchase #7 $122.66   11.753%   $137.0762   $137.08   $137.10   $137.10  
  7 purchase totals = $702.30       $757.7882   $757.80   $757.80   $757.70