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 $21.91   4.372%   $22.8679   $22.87   $22.85   $22.90  
  Purchase #2 $5.32   9.477%   $5.8242   $5.82   $5.80   $5.80  
  Purchase #3 $172.42   9.252%   $188.3723   $188.37   $188.35   $188.40  
  Purchase #4 $123.79   9.742%   $135.8496   $135.85   $135.85   $135.80  
  Purchase #5 $183.26   10.207%   $201.9653   $201.97   $201.95   $202.00  
  Purchase #6 $171.42   3.424%   $177.2894   $177.29   $177.30   $177.30  
  Purchase #7 $89.72   9.158%   $97.9366   $97.94   $97.95   $97.90  
  7 purchase totals = $767.84       $830.1053   $830.11   $830.05   $830.10