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 $230.56   13.363%   $261.3697   $261.37   $261.35   $261.40  
  Purchase #2 $34.72   12.774%   $39.1551   $39.16   $39.15   $39.20  
  Purchase #3 $74.74   12.251%   $83.8964   $83.90   $83.90   $83.90  
  Purchase #4 $140.89   12.854%   $159   $159.00   $159.00   $159.00  
  Purchase #5 $58.93   13.348%   $66.796   $66.80   $66.80   $66.80  
  Purchase #6 $96.57   3.590%   $100.0369   $100.04   $100.05   $100.00  
  Purchase #7 $48.52   11.408%   $54.0552   $54.06   $54.05   $54.10  
  7 purchase totals = $684.93       $764.3093   $764.33   $764.30   $764.40