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 $243.86   12.256%   $273.7475   $273.75   $273.75   $273.70  
  Purchase #2 $58.97   4.850%   $61.83   $61.83   $61.85   $61.80  
  Purchase #3 $160.71   8.308%   $174.0618   $174.06   $174.05   $174.10  
  Purchase #4 $50.68   7.746%   $54.6057   $54.61   $54.60   $54.60  
  Purchase #5 $249.53   8.723%   $271.2965   $271.30   $271.30   $271.30  
  Purchase #6 $138.96   9.514%   $152.1807   $152.18   $152.20   $152.20  
  Purchase #7 $250.50   10.609%   $277.0755   $277.08   $277.10   $277.10  
  7 purchase totals = $1,153.21       $1,264.7977   $1,264.81   $1,264.85   $1,264.80