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 $137.37   9.115%   $149.8913   $149.89   $149.90   $149.90  
  Purchase #2 $171.25   9.201%   $187.0067   $187.01   $187.00   $187.00  
  Purchase #3 $25.93   11.579%   $28.9324   $28.93   $28.95   $28.90  
  Purchase #4 $171.97   4.606%   $179.8909   $179.89   $179.90   $179.90  
  Purchase #5 $100.59   8.198%   $108.8364   $108.84   $108.85   $108.80  
  Purchase #6 $183.79   4.178%   $191.4687   $191.47   $191.45   $191.50  
  Purchase #7 $79.55   10.799%   $88.1406   $88.14   $88.15   $88.10  
  7 purchase totals = $870.45       $934.1670   $934.17   $934.20   $934.10