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 $237.49   8.764%   $258.3036   $258.30   $258.30   $258.30  
  Purchase #2 $55.28   8.207%   $59.8168   $59.82   $59.80   $59.80  
  Purchase #3 $176.86   12.384%   $198.7623   $198.76   $198.75   $198.80  
  Purchase #4 $70.36   10.182%   $77.5241   $77.52   $77.50   $77.50  
  Purchase #5 $5.80   13.413%   $6.578   $6.58   $6.60   $6.60  
  Purchase #6 $249.57   11.159%   $277.4195   $277.42   $277.40   $277.40  
  Purchase #7 $225.93   13.460%   $256.3402   $256.34   $256.35   $256.30  
  7 purchase totals = $1,021.29       $1,134.7445   $1,134.74   $1,134.70   $1,134.70