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 $115.30   4.192%   $120.1334   $120.13   $120.15   $120.10  
  Purchase #2 $9.79   7.106%   $10.4857   $10.49   $10.50   $10.50  
  Purchase #3 $23.83   13.803%   $27.1193   $27.12   $27.10   $27.10  
  Purchase #4 $133.85   10.363%   $147.7209   $147.72   $147.70   $147.70  
  Purchase #5 $22.53   11.748%   $25.1768   $25.18   $25.20   $25.20  
  Purchase #6 $100.39   10.584%   $111.0153   $111.02   $111.00   $111.00  
  Purchase #7 $17.65   10.734%   $19.5446   $19.54   $19.55   $19.50  
  7 purchase totals = $423.34       $461.1960   $461.20   $461.20   $461.10