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 $113.41   7.232%   $121.6118   $121.61   $121.60   $121.60  
  Purchase #2 $12.79   11.733%   $14.2907   $14.29   $14.30   $14.30  
  Purchase #3 $28.94   8.219%   $31.3186   $31.32   $31.30   $31.30  
  Purchase #4 $131.36   7.358%   $141.0255   $141.03   $141.05   $141.00  
  Purchase #5 $10.65   6.288%   $11.3197   $11.32   $11.30   $11.30  
  Purchase #6 $105.50   11.802%   $117.9511   $117.95   $117.95   $118.00  
  Purchase #7 $181.81   11.346%   $202.4382   $202.44   $202.45   $202.40  
  7 purchase totals = $584.46       $639.9556   $639.96   $639.95   $639.90