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 $120.94   11.454%   $134.7925   $134.79   $134.80   $134.80  
  Purchase #2 $194.58   4.708%   $203.7408   $203.74   $203.75   $203.70  
  Purchase #3 $51.97   8.122%   $56.191   $56.19   $56.20   $56.20  
  Purchase #4 $25.37   12.811%   $28.6202   $28.62   $28.60   $28.60  
  Purchase #5 $168.95   10.259%   $186.2826   $186.28   $186.30   $186.30  
  Purchase #6 $27.65   12.158%   $31.0117   $31.01   $31.00   $31.00  
  Purchase #7 $163.74   12.503%   $184.2124   $184.21   $184.20   $184.20  
  7 purchase totals = $753.20       $824.8512   $824.84   $824.85   $824.80