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 $107.35   11.818%   $120.0366   $120.04   $120.05   $120.00  
  Purchase #2 $165.42   9.774%   $181.5882   $181.59   $181.60   $181.60  
  Purchase #3 $182.25   13.493%   $206.841   $206.84   $206.85   $206.80  
  Purchase #4 $191.49   12.349%   $215.1371   $215.14   $215.15   $215.10  
  Purchase #5 $86.81   12.229%   $97.426   $97.43   $97.45   $97.40  
  Purchase #6 $126.60   11.293%   $140.8969   $140.90   $140.90   $140.90  
  Purchase #7 $32.53   11.516%   $36.2762   $36.28   $36.30   $36.30  
  7 purchase totals = $892.45       $998.2020   $998.22   $998.30   $998.10