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 $29.60   6.404%   $31.4956   $31.50   $31.50   $31.50  
  Purchase #2 $103.95   5.594%   $109.765   $109.77   $109.75   $109.80  
  Purchase #3 $116.68   13.500%   $132.4318   $132.43   $132.45   $132.40  
  Purchase #4 $128.80   5.145%   $135.4268   $135.43   $135.45   $135.40  
  Purchase #5 $193.52   5.721%   $204.5913   $204.59   $204.60   $204.60  
  Purchase #6 $44.46   9.483%   $48.6761   $48.68   $48.70   $48.70  
  Purchase #7 $233.84   4.232%   $243.7361   $243.74   $243.75   $243.70  
  7 purchase totals = $850.85       $906.1227   $906.14   $906.20   $906.10