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 $54.67   11.522%   $60.9691   $60.97   $60.95   $61.00  
  Purchase #2 $100.70   7.187%   $107.9373   $107.94   $107.95   $107.90  
  Purchase #3 $150.52   10.816%   $166.8002   $166.80   $166.80   $166.80  
  Purchase #4 $46.33   12.585%   $52.1606   $52.16   $52.15   $52.20  
  Purchase #5 $5.75   9.238%   $6.2812   $6.28   $6.30   $6.30  
  Purchase #6 $154.82   8.211%   $167.5323   $167.53   $167.55   $167.50  
  Purchase #7 $16.73   5.523%   $17.654   $17.65   $17.65   $17.70  
  7 purchase totals = $529.52       $579.3347   $579.33   $579.35   $579.40