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 $30.48   4.870%   $31.9644   $31.96   $31.95   $32.00  
  Purchase #2 $139.49   7.777%   $150.3381   $150.34   $150.35   $150.30  
  Purchase #3 $244.76   10.479%   $270.4084   $270.41   $270.40   $270.40  
  Purchase #4 $167.55   13.291%   $189.8191   $189.82   $189.80   $189.80  
  Purchase #5 $25.74   5.665%   $27.1982   $27.20   $27.20   $27.20  
  Purchase #6 $129.91   7.515%   $139.6727   $139.67   $139.65   $139.70  
  Purchase #7 $9.67   10.281%   $10.6642   $10.66   $10.65   $10.70  
  7 purchase totals = $747.60       $820.0651   $820.06   $820.00   $820.10